1

1 United States Department of Commerce

I 1

^^^^

I Technology Administration ' ^"^^^^ ' National Institute of Standards and Technology

Technical Digest

Symposium on Optical Fiber

Measurements, 1998

Sponsored by the National Institute of Standards and Technology

in cooperation with the IEEE Lasers and Electro-Optics Society

and the Optical Society of America
I

I

0

I
NIST Special Publication 930

Technical Digest—Symposium on Optical

Fiber Measurements, 1998

Digest of a symposium sponsored by the

National Institute of Standards and Technology

in cooperation with the IEEE Lasers and Electro-Optics Society

and the Optical Society of America

Edited by

G.W. Day

D.L. Franzen

P. A. Williams

Electronics and Electrical Engineering Laboratory

National Institute of Standards and Technology

Boulder, Colorado 80303-3328

September 1998

U.S. DEPARTMENT OF COMMERCE, William M. Daley, Secretary

TECHNOLOGY ADMINISTRATION, Gary R. Bachula, Acting Under Secretary for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Raymond G. Kammer, Director

TES
/

National Institute of Standards

and Technology

Special Publication 930

Natl. Inst. Stand. Technol.

Spec. Publ. 930

196 pages (September 1998)

C0DEN:NSPUE2

U.S. Government Printing Office

Washington: 1998

For sale by the Superintendent of

Documents

U.S. Government Printing Office

Washington, DC 20402-9325
Preface

This year marks the 10th biennial Symposium on Optical Fiber Measurements, held

September 15-17, 1998 at the laboratories of the National Institute of Standards and

Technology in Boulder, Colorado.

Over these last 20 years, the SOFM has been a good indicator of the issues that concern

the fiber optic measurement community. At first glance, the second day's multimode

fiber measurement talks may seem like a trip back in time to an SOFM from the eighties.

However, recent events have brought multimode fiber issues back into the measurement

arena. Polarization-mode dispersion (PMD) continues to be a topic of much interest with

two full sessions devoted to a variety of subtopics in the field. Fiber geometry has

several contributions, as does the broad topic of fiber mapping with length (including

such parameters as chromatic dispersion and polarization properties).

This year's Symposium consists of 44 papers (10 invited and 34 contributed), and

continues to show a strong international participation, with two-thirds of the papers

originating outside the United States.

G.W. Day

D.L. Franzen

P.A. Williams

Boulder, Colorado

September 1998

Except where attributed to NIST authors, the content of individual sections of this volume has not been reviewed or edited by the National Institute of Standards and Technology. NIST therefore accepts no responsibility for comments or recommendations therein. The mention of trade names in this volume is in no sense an endorsement or recommendation by the National Institute of Standards and Technology.

iii
SYMPOSIUM COMMITTEE

D.L. Franzen, NIST, General Chair

G.W. Day, NIST, Program Co-Chair

P.A. Williams, NIST, Program Co-Chair

M. Artiglia, CSELT

A. Barlow, EG&G Fiber Optics

J. Benson, NPL

S.C. Fleming, University of Sydney

N. Gisin, University of Geneva

T.A. Hanson, Corning

B.L. Heffner, Hewlett-Packard

R.B. Kummer, Lucent Technologies

P.S. Lovely, PK Technology

H. Nagai, Anritsu

W.A. Reed, Lucent Technologies

L.A. Reith, Bellcore

D.K. Roland, SpecTran

G.W. Schinn, EXFO

K. Takada, NTT

iv
CONTENTS

PREFACE iii SYMPOSIUM COMMITTEE iv

DWDM Component Characterization (invited)

Bruce Nyman, JDS Fitel : 1

Accurate Polarization Dependent Loss Measurement and Calibration Standard Development

R.M. Craig, S.L. Gilbert, P.D. Hale, NIST 5

Measurement of Wide-Bandwidth Gain-Flat Fiber Amplifiers (invited)

Paul F. Wysocki, Lucent Technologies 9

Polarisation Mode Dispersion in Chirped Fibre Bragg Gratings (invited)

David Ives, NPL 15

Measurement of Polarisation Mode Dispersion in Fibre Amplifiers

C. Geiser, B. Huttner, N. Gisin, University of Geneva; R. Caponi, M. Potenza,

M. Schiano, M. Artiglia, CSELT; I. Joindot, France Telecom/CNET 19

Narrowband Measurements of Polarization-Mode Dispersion Using the Modulation

Phase Shift Technique

P. A. Williams, NIST; A.J. Barlow, C. Mackechnie, EG&G Fiber Optics;

J.B. Schlager, NIST 23

Impact of Signal Wavelength and Input State-of-Polarization on PMD Induced Pulse

Distortion in > 10 Gbit/s Transmission Systems

Ralph Leppla, Arnold Mattheus, Deutsche Telekom AG 27

Repeated Measurement of Polarisation Mode Dispersion in Installed Cables

Adam Kapovits, Hungarian Telecommunications Company, Ltd 31

Twin-Photon Techniques for Fiber Measurements (invited)

N. Gisin, J. Brendel, H. Zbinden, University of Geneva; A. Sergienko, A. Muller,

Boston University 35

Comparison Between Interferometric and Polarimetric PMD Measurement Techniques

Based on Fiber Impulse Response

Marco Schiano, CSELT 41

Determination of Small PMD Values via the Poincare Sphere Method Using Wavelength

Scanning Data

N. Cyr, EXFO 45

V
Polarisation-Induced Pulse Spreading in Birefringent Optical Fibres with Zero

Differential Group Delay

B. Huttner, B. Gisin, N. Gisin, University of Geneva 49

Temperature Dependence of PMD Measurements

S. Corbeil, M. O' Sullivan, NORTEL 53

Measurement of Two Dimensional Axial Stress Profiles of Optical Fibres with High

Spatial Resolution

K.W. Raine, K&V Optics; A.J. Parker, NPL 57

Biased Perturbation Method for Index Profiling Single-Mode Fibers from Near-Field

and Far-Field Data

N.H. Fontaine, M. Young, NIST 61

High Precision Fiber Group Refractive Index Measurement Using an Interferometric Method

Gisella Marradi, Pier Giuseppe Peretta, Susanna Cattelan, SIRTI 65

Are the Formulas for Mode-Field Diameter Correct?

Ronald C. Wittmann, Matt Young, NIST 69

Simultaneous Measurement of Relative Outer Diameter along Fiber Axis: nm Resolution

in Diameter and mm Coverage in Length

Andrew W. Poon, Richard K. Chang, Yale University 73

Measurement of the Impact of Fiber Nonlinearities on High Data Rate, DispersionManaged WDM Systems (invited)

Sebastien Bigo, Michel Chbat, ALCATEL 77

Highly Accurate Nonlinear Coefficient Measurements by SPM Method for DSFs

and Large Effective Area Fibers at 1.55 fxm

Yoshinori Namihira, KDD R&D Laboratories 83

Monitoring of Optical Signal Quality Using Sum-Frequency-Generation Optical Sampling

Ippei Shake, Hidehiko Takara, Satoki Kawanishi, Yoshiaki Yamabayashi,

NTT Optical Network Systems Laboratories 87

High Resolution Reflectometry for Diagnosis of Optical Devices by Synthesis of Optical

Coherence Function (invited)

Kazuo Hotate, Takashi Saida, University of Tokyo 91

Distributed-Gain Measurements of Erbium-Doped Fibers

J.P. von der Weid, A.O. Dal Forno, J. A. Pereira da Silva, R. Passy, Pontificia

Universidade Catolica do Rio de Janerio; M.R. Xavier de Barros, CpqD-Telebras;

B. Huttner, N. Gisin, University of Geneva 97

vi
Polarisation OFDR for Measurements of Birefringence and Polarisation Mode Coupling

Lengths in Optical Fibres

B. Huttner, J. Reecht, N. Gisin, University of Geneva; R. Passy, J. P. von der Weid,

Pontificia Universidade Catolica do Rio de Janerio 101

Beat-Length Measurement in Highly Mode-Coupled Single-Mode Fibers by Means of

Fourier Transform of Backscattered Signal

Fabrizio Corsi, Andrea Galtarossa, Luca Palmieri, DEI Univesita di Padova 105

Spun Fibre Parameter Extraction Using Polarimetric Optical Time Domain Reflectometry

J.G. Ellison, A.S. Siddiqui, University of Essex 109

Characterizing Multimode Fiber Bandwidth for Gigabit Ethernet Applications (invited)

Michael J. Hackert, Coming 113

Effect of Launch Conditions on Power Penalties in Gigabit Links Using 62.5 yum

Core Fibers Operating at Short Wavelength

Petar Pepeljugoski, IBM; John Abbot, Corning; Jim Tatum, Honeywell Microswitch ..119

Improved Performance in Short-Haul Multimode Fibre Systems by Using Low-Order

Launch Conditions

Robert S. Billington, University of Leeds; Jerry M. Benson, Timothy C.E. Jones,

NPL; John C.C. Nelson, University of Leeds 123

Differential Mode Delay Measurements on Multimode Fibers in the Time and

Frequency Domains

J.B. Schlager, D.L. Franzen, NIST 127

Ultra-high Resolution Two Dimensional Differential Mode Delay Measurements of

Multimode Fibers

Constantine T. Markos, Alfred D. Ducharme, Gerry Wyntjes, Visidyne 131

Intermodal Dispersion and Mode Coupling in Perfluorinated Graded-Index Plastic Optical Fiber

Michael Dueser, W.R. White, W.A. Reed, Lucent Technologies; Tsuyoshi Onishi,

Asahi Glass Company 135

Measurement of Skew in GI MM Optical Ribbon Fibers

Masateru Tadakuma, Osamu Aso, Shu Namiki, Furukawa Electric Company 139

Gratings for WDM Systems - Review of Applications and Critical Measurements (invited)

Peter A. Krug, John Canning, Dmitrii Yu. Stepanov, OFTC 143

Real-Time Characterisation of Fibre Grating Fabry-Perots and Phase-Shifted Gratings

with 100 kHz Resolution

D. Yu. Stepanov, J. Canning, Z. Brodzeli, OFTC 149

vii
OTDR-Like Dispersion-Map Measurements (invited)

Jiirgen Gripp, Linn F. Mollenauer, Lucent Technologies 153

Measurement of the Spatial Distribution of the Zero-Dispersion Wavelength in DS

Fibers by Bidirectional Linear Reflectometry

A. Rossaro, M. Schiano, CSELT; M. Dainese, F. Lovisolo, Politechnico di Torino ... 159

A New Technique for Zero-Dispersion Wavelength Mapping in Single Mode Fiber

with High Spatial Resolution

L Brener, D.D. Lee, P.P. Mitra, D. Philen, D.J. Thomson, Lucent Technologies 163

Analysis of Second Order Chromatic Dispersion Variation in Optical Fiber under

Large Stretching

L.M. Simohamed, J.L. Auguste, J. Rioublanc, J.M. Blondy, F. Reynaud, IRCOM 165

Fiber Dispersion Measurements Using a Supercontinuum Fiber Laser

F. Koch, S.V. Chernikov, J.R. Taylor, Imperial College 169

Planar Components for Dense-WDM (invited)

J. Amin, R.O. Maschmeyer, A. Beguin, C. Lerminiaux, D. Trouchet, N. Boos,

C. Nicolas, Corning 173

2.5 Gb/s PRBS Extinction Ratio "Eye Diagram" Measurements of Biased and Bias Free

Lithium Niobate OC-48 Modulators

David Williams, Adnan Akhtar, Dan Attanasio, David R. Maack, Greg McBrien,

Uniphase Telecommunication Products 175

Dependence of the Composition of InAsi.^Sbx on the Mole Fraction of In in the Melt Using LPE

Vishal Chatrath, S.J. Chua, G. Karunasiri, National University of Singapore; Y. Mao,

Lancaster University 179

Active Mode-Hop Suppression in External Cavity Lasers

Shigenori Mattori, Takanori Saitoh, Shigeru Kinugawa, Koichiro Miyagi,

Anritsu Corporation 183

Author Index 187

viii
DWDM Component Characterization

Bruce Nyman

JDS Fitel

Introduction

Wavelength division multiplexing has enabled tremendous growth in fiber optic communication

systems. Initial WDM systems used two channels at 1.3 and 1.5 jam while today's systems pack up

to 96 channels into the 35 nm bandwidth provided by erbium doped fiber amplifiers. As the

channel spacing has approached 50 GHz the component characterization requirements have

increased. A typical system contains many devices such as isolators, taps, and wavelength division

multiplexers. The key component in today's WDM systems is the demultiplexer. Its performance

limits the overall system capacity and design margin. Therefore, we will examine the measurement

requirements of this device. Other devices will have similar measurement needs.

The function of the demultiplexer is to separate the channels at the end of a transmission system.

There are currently four different technologies used; dielectric filters, fiber Bragg gratings,

Difraction Gratings and waveguide based devices. Devices for 100 GHz systems have been

demonstrated in each of these technologies. The best technology for 50 GHz devices is still not

clear. A key issue for 50 GHz systems will be the stability of the lasers.

The number of channels in the system will be determined by the channel spacing of the demultiplexer. As we will see later the allowable channel spacing is a combination of the passband

width and the out of band isolation. For systems without amplifiers the insertion loss will limit the

overall system power budget.

Insertion Loss Based Measurements

The key measurement for characterizing DWDM devices is insertion loss versus wavelength. Many

of the device parameters are based on an interpretation of the insertion loss data. The requirements

for the insertion loss measurement are very stringent for the demultiplexer.

As an example we consider the case of an 80 channel demultiplexer for a 50 GHz system.

The first requirement for this measurement is the wavelength range. For the 80 channel system the

wavelength range is at least 35 nm. The accuracy of the wavelength should be better than 0.01 nm. The dynamic range required will be at least 50 dB. This is needed to measure the performance of

the demultiplexer at adjacent channels. Finally, the most difficult requirement is slope accuracy.

For 50 GHz (0.4 nm) spacing, the channel width measured at the 1 dB down points might be 0.2 nm. At the edge of the next channel. 0.2 nm away, the system needs the loss of this channel to be at

least 30 dB. Assuming a linear slope, the slope accuracy must be better than 145 dB/nm. I real devices the slope will be even sharper.

Once the insertion loss versus wavelength data is available it can be analyzed to obtain the desired

device parameters. Below, these parameters are discussed.

Center Wavelength: How should the nominal center wavelength be defined? Ideally, the center wavelength is the designated ITU wavelength. However, an alternative definition might be the

1
center of a nominal bandwidth. For example, the center of two points measured 1 dB down from

the minimum insertion loss could be used. This approach is useful if the passband is not flat. Bandwidth: The bandwidth is specified relative to the minimum insertion loss point. The exact bandwidth is then expressed as the number of dBs below the minimum insertion loss point. For

example, a 1 dB bandwidth might specify the usable channel bandwidth, while a 30 dB bandwidth

would lead to the limits on adjacent channel spacing.

Isolation: This isolation is typically defined as the amount of light from other channels that will be

transmitted through the device. This is sometimes divided into two components, the power from the

adjacent channel and the power from all the other channels. This is also sometimes referred to as

crosstalk. One interesting aspect of this requirement is that the entire system wavelength band must

be examined. Indeed what is really required is the sum of all the powers at all other channels. This

is especially true for fiber Bragg gratings and waveguide devices.

Return Loss: The reflectivity of the device should be measured across the entire wavelength band.

This is the integrated total return loss not a spatially resolved measurement.

In measuring these quantities one must consider the degree of polarization and the polarization

state of the light source used. All the measurements discussed above are polarization dependent.

Therefore it will be necessary to determine the variations that occur with polarization. This leads to some interesting questions about definitions. There are no standards that define the concepts of

polarization dependent bandwidth and crosstalk.

For example, lets consider the bandwidth. One way to determine the worst case bandwidth would

be to measure the polarization dependent loss at each wavelength and plot the maximum and

minimum loss. The bandwidth would then be defined as the maximum or minimum loss envelope

as required. The minimum loss envelope might be used for the 30 dB bandwidth while the

maximum loss envelope might be used for the 1 dB channel passband. This approach makes no

assumptions about how polarization dependent loss at each wavelength varies with the state of

polarization at adjacent wavelengths. In real systems this may be required due to the effect of PMD

on the state of polarization. So the question is, what is this the proper way to define the

polarization dependent bandwidth?

Once techniques to measure these quantities over wavelength and polarization are available making

measurements for reliability testing are possible. Typically these involve characterizing the device's

performance over a temperature range and a humidity range. These tests are defined in documents

such as Bellcore 1 209 and 1 22 1

Other Measurement Needs

There are two other measurements that are needed for DWDM components, chromatic dispersion,

and polarization mode dispersion (PMD). The chromatic dispersion must be measured both at the

center of the passband and at the edges. The chromatic dispersion at the edges of the passband can

effect adjacent channels in add/drop configurations. The chromatic dispersion has been examined

for various filter types.' For devices with narrow passbands the traditional measurement methods^

will not suffice. The measurement method must be modified for the narrow passbands.

PMD can be a problem for many DWDM components. Typically devices such as isolators,

circulators and the erbium fiber itself can have significant PMD. Demultiplexers and other narrow

2
band devices must be characterized for PMD. This will require measurement techniques modified

for the narrow bandwidths. In addition, there are higher order PMD effects that need to be

measured.

Measurement Methods

In DWDM systems there is a large variation in the types of devices used. Some devices, such as

isolators and taps have little spectral information, and thus need simple measurement methods.

Devices for optical supervisory channel add and drop functions are more complex. Demultiplexers

and channel add drop devices will require more complicated measurement methods. These devices

need both high wavelength resolution and accuracy, on the order of 5 pm. At the same time,

measurements must be made over the entire wavelength range.

The simplest measurement technique will use a broadband noise source and an optical spectrum

analyzer (OSA). The broadband noise source is unpolarized with a short coherence length. This

approach provides a fast way to measure insertion loss versus wavelength. This approach can be

used to measure isolation and return loss. However, measurements of PDL will require additional

equipment.

The more advanced measurements require a tunable laser source. This source has a narrow

linewidth (< 10 MHz), and is polarized. The laser can be tuned in 1 pm steps over the entire wavelength range. The combination of a tunable laser with either a power meter or an OSA

provides a high resolution measurement with the needed wavelength and power accuracy. The only

drawback of this method is scan speed. Each measurement takes about a few seconds. Thus the

time for covering the entire 35 nm span with 5 pm resolution could be a few hours. This is what

would be needed to measure isolation in a demultiplexer. Adding polarization dependent

measurements will increase the measurement time. Improving the measurement time has been the

topic of recent developments. A system that reduces the measurement time to 3 seconds for the

entire wavelength range with 3 pm resolution will be discussed.

The measurement methods for chromatic dispersion and PMD have similar issues with the

wavelength characteristics of the devices. For the narrow bandwidth devices a tunable laser must

be used. For chromatic dispersion a phase sensitive technique is typically used. Here the tunable

laser is modulated and the phase delay to the device is measured. For PMD the tunable laser is used as the light source for a Jones matrix PMD measurement method.

Conclusions

The variety of devices used in DWDM systems lead to various measurement requirements and

methods. Demultiplexers have the most difficult measurement requirements and test methods.

Common definitions for the measurements should be developed. Given the need for high resolution

measurements over the entire 35 nm bandwidth new testing methods will be required.

References

' "Optimal Dispersion of Optical Filters for WDM Systems," G. Lenz, B. J. Eggleton, C. K.

Madsen, C. R. Giles, and G. Nykolak, IEEE Photon. Tech. Lett., 567(1998).

^ "Fiber Optic Test and Measurement," Dennis Derickson, editor. Prentice Hall, chapter 12.

3
!

i 4
Accurate Polarization Dependent Loss Measurement

and Calibration Standard Development

R. M. Craig, S. L. Gilbert, and P. D. Hale

National Institute of Standards and Technology, 325 Broadway, Boulder, CO 80303

Abstract - We have implemented an automated,

nonmechanical approach to the measurement of

polarization dependent loss. We use a

deterministic fixed-states method to obtain

Mueller matrix elements from intensity

measurements at specific polarization states. Voltage-modulated liquid-crystal variable

retarders set the input polarization states. Synchronous detection is employed to increase the

signal-to-noise ratio of the system and thereby

allow a measurement resolution of better than

0.001 dB. We present polarization-dependent loss measurements from 0.0016 to 0.56 dB at 1550 nm

to verify performance. We also present results from potential artifact calibration standards of an

all-fiber design.

Introduction

Polarization dependent loss (PDL) is defined

as 101og(rmax/rmin) (dB) whcre T is transmittance

taken over the entire polarization-state space. Polarization dependent loss is usually

characterized as a localized component effect as opposed to the distributed nature of polarization

mode dispersion. PDL measurement methods can

be divided into three categories, all of which are represented by commercial instrumentation:

deterministic fixed-states, deterministic all-states,

and pseudorandom all-states [1]. We have

implemented a deterministic fixed-states

technique. Our goal is to establish the capability

to measure PDL with a resolution finer than 0.001

dB and to determine the absolute accuracy of our method. In this paper we summarize an approach

to polarization dependent loss measurement that employs a nonmechanical technique capable of

synchronous detection. This system has been

described more fully in [2]. We also present new

results from an all-fiber PDL artifact reference

that is potentially useful as a transfer standard.

This may lead to a Standard Reference Material

for the calibration of commercial instrumentation.

Because PDL is always greater than or equal

to zero, noise in the PDL measurement system can degrade the sensitivity to very small PDL values

(where the PDL is less than the system's singlemeasurement noise). Sensitivity to these small

PDL values cannot be improved by increasing the

number of measurements; the average of multiple

PDL measurements will always yield a positive

value that is proportional to the singlemeasurement noise. However, synchronously

averaging measured intensities in a deterministic

fixed-states method can improve sensitivity due to the increase in the signal-to-noise ratio. We implemented our system using voltagemodulated liquid-crystal variable retarders

(LCVR) and synchronous detection. A different

LCVR approach has also been reported [3]. However, in our case, modulation of the

polarization state allows differential measurement

as well as an improvement in sensitivity due to synchronous time averaging.

Concept

Our method, which we call the Mueller-Stokes

technique, is a variation on a matrix technique

developed by Favin et al. [4], and relies solely on power measurements at specific polarization

states. In this deterministic fixed-states method,

four well-defined polarization states are necessary

to determine the first-row Mueller matrix elements

of a component. The global polarization

dependence of transmittance can then be

determined from these four elements.

Because the measurement depends only on the

relative coordinates of the four states, the only

requirements on the set are that they maintain

relative angular separations of 90° about the origin

of the polarization (Poincare) sphere. This

implies that constant intervening retardance, as represented by rotations of the sphere, will have no net effect. A representative sample of states is shown in Fig. la, while a rotation of those states is

5
Figure la: Initial Poincare Figure lb: LCVR trajectories trajectories of the LCVR following birefringent

pair over a measurement. displacement

seen in Fig. lb. In our system, the polarization states are produced by two LCVR units in series. These

modulate the polarization state of a low-coherence

polarized beam. The effect of the pair is to produce final polarization states equivalent to those produced by a quarter-wave and half-wave

retarder combination undergoing independent

rotations. Following generation of the four

polarization states (with average powers h, /a), the light is transmitted by a single mode fiber with

arbitrary but relatively stable birefringence

through the device under test (DUT). It proceeds

to a polarization-insensitive detector which

measures the four time-averaged output powers

(/i, h)- We measure the time-averaged input

powers (/a, . .

. , h) in the same way with the DUT

removed to establish a baseline system response.

The four first-row Mueller matrix elements

(mil,..., mi4) are combinations of ratios of these

powers [2]. Information about the global

transmittance extrema are contained in the first row matrix elements. The transmittance extrema,

Tinin and Tlnax; are

Tnin =^11 -V^12+^13+^14 • (1)

Polarization dependent loss then follows as defined above.

The essential advantage of this technique is nonmechanical retardance modulation, which

allows rapid synchronous time averaging in a low

noise environment at a frequency set by an

external clock. Synchronous time averaging

improves the signal-to-noise ratio and can be

applied in different ways depending on the method

of signal acquisition chosen.

Implementation

Figure 2 shows a schematic of the

measurement system. Our system consists of five

major sections: source, LCVR cavity, detector,

boxcar averager and control computer. We

connect the polarization-modulated light directly

to the DUT without any intervening components

other than single-mode fiber and fiber connectors. Measurements at each polarization state are Open Beam Cavity

'

I

'

I

I Polarizer LCVR 1 LCVR 2

^

I 1 I °° —I r-^—I n I Detector

I \p^^

Launcher

|

I Source |

I _ _ Monitor _ ^

Figure 2: Simplified block diagram of the LCVR PDL

measurement system.

limited to a fixed number N of modulation cycles

at clock frequencies of 3.5 Hz to 10 Hz. Each of

the three congruent line segments in Fig. 1 originating from the vertex represents the

modulation path between two Mueller-Stokes

states and therefore a signal to be sampled. This

signal is routed to two averager channels, and

each channel has a delay set to position the

respective gates on alternate periods. To the extent that we can control birefringence drift during the sampling of each polarization state, resolution increases as VN. In addition, we use

differential measurements to eliminate commonmode drift. To obtain the proper Stokes powers,

each of the three difference measurements must be

subtracted from the absolute power of the central

polarization state. We measure the central state power by chopping the source at the clock

frequency.

Uncertainties

A detailed analysis of the uncertainties in our PDL measurement system shows several dominant

uncertainty sources. We list them in order of

decreasing importance. Values listed correspond

to one standard uncertainty (la) about the signal

power Isig.

6
Polarization state uncertainty. Polarization

state accuracy depends on the accuracy of the

calibrating polarimeter as given by the

manufacturer. Value: ±0.83% of Isig. Retarder temperature dependence. We

quantify temperature dependence primarily in terms of its effect on the LCVR elements. A

worst-case temperature variation of 0.5 °C is used'

to derive the uncertainty in signal power due to variation in the polarization state of ±0.2%.

Value: ±0.12% of Isig System internal PDL variation. The system

retardance elements contribute their own PDL

(0.02 dB) to that of the DUT. If this value is constant during both phases of the measurement, it

is cancelled in the ratios. Our quoted PDL

variation was derived from the maximum

observed slope of the internal PDL over 15 minute

intervals. Value: ±0.12% of Lig. Propagated measurement system uncertainty.

We have propagated the uncertainties above

inherent to the measurement system in a model of

the nonideal system. The combined total PDL

deviations are then calculated from the defining

equations [2] to be ±2.1%. Value: ±2.1% of PDL.

PDL measurement repeatability. The

repeatability given is one standard deviation (la)

of repeated undisturbed measurements following

the initial baseline measurement. By undisturbed,

we mean that one sample follows another with no

connector disconnection in between. This value is the effective system noise. Value: ±0.0008 dB.

Combined standard uncertainty. The effects are assumed uncorrelated, so we combine the

above results using the root-sum-of-squares (RSS)

method as (±0.0008 dB & 2.1% of PDL) RSS.

A significant source of uncertainty, which was

not inherent to the LCVR system, was variations

in measured PDL of DUT artifacts with

connectorized pigtails following disconnection

and reconnection. This could be explained by

connector alignment errors. For those artifacts we

add an additional ±0.0029 dB uncertainty. In our

system, we took care to minimize fiber motion

during connector handling. However, if this is not done, one could introduce errors from alterations

in the relative orientations of system, DUT, and

connector PDL axes due to shifts in pigtail

birefringence

Stability in system birefringence is important

both during and between each set of power

measurements in any fixed-states scheme. Large

intermediate instabilities or drifts could lead to significant uncertainty. While the effects can be

modeled, it is best to simply minimize the

instability. An advantage of our technique is that measurements can be performed quickly, so the

stability criterion is easily satisfied. We compared measurements between the

fixed-states technique and the commonly used

polarization-scanning (or all-states) method. The

all-states technique relies on pseudorandom

sampling of all polarization states with a total uncertainty that is proportionally reduced by

increasing measurement time and/or scanning

speed. For PDL < 0.1 dB, uncertainties in the

range of ± 3% to ±5% of PDL are claimed for a 30

second measurement [5].

Results

The primary artifact chosen to test the

performance of the system provides a calculable

PDL of moderate accuracy. It consists of an open

beam launcher/collimator (a cleaved section of

single-mode fiber and objective lens) followed by

4^ 0.2 <

Artifact Measured via Mueller-Stokes

Artifact Predicted Values

Artifact Measured via All-States

0.0016 dB LCVR offset 0.0035 dB All-Stales offset

Incidence Angle {°)

Figure 3: Open-beam artifact data from both the Mueller- Stokes and all-states systems. Error bars represent total uncertainty. The solid line is the calculated PDL value.

a polished BK7 glass cube. We mount the cube

on a rotation stage with 5' resolution. The InGaAs

detector (which includes a ceramic depolarizer) is translated to compensate for beam displacement

following rotation. Calculated PDL (Fresnel) and

measured (Mueller-Stokes) values as a function of

input angle are presented in Fig. 3 and are in good

7
agreement. Our residual system PDL was

measured and accounted for as a 0.0016 ± 0.0001

dB offset at normal incidence, where the

uncertainty quoted is the statistical uncertainty of

the mean with 95% confidence interval (2<jHN).

This nominally left only the dual glass/air

interfaces to produce PDL.

To test the measurement resolution, a series of

Mueller-Stokes measurements were performed by

alternating between 15° and 15°5' and measuring

the PDL difference. We found the average of

those measurements, 0.0008 ±0.0003 dB, to be in good agreement with the predicted value.

All-Fiber PDL Artifact

We have constructed a series of all-fiber PDL

artifacts based on fusion-spliced sections of

single-mode, polarizing, and multimode fiber as shown in Fig. 4. These artifacts show promise for use as calibration transfer standards. We use hotwire splicing to produce clean joints between

dissimilar fibers. The input is a short

Input Section PDL Section Output Section (singlemode) (polarizing fiber) (multimode depolarizer) <1 m 2 - 10 mm >20 m

Figure 4: Schematic of the prototype all-fiber PDL artifact.

single-mode section for accurate coupling to system leads. The artifact section is a few

millimeters of polarizing fiber (18 dB/m rejected

mode extinction) that provides stable PDL. We

have generated PDL in the range of 0.063 to 0.230

dB with lengths of 5 to 17 mm, where PDL

increases with polarizing fiber length. Splices can modify artifact behavior, possibly by introducing

additional PDL or changing the rejected-mode

extinction. The output section is composed of

several meters of multimode fiber to minimize

losses due to aperture mismatch and depolarize

the signal, thereby reducing uncertainty due to detector PDL. These artifacts provide good

temperature stability over the range of 0 to 40 °C.

We have measured a PDL temperature

dependence of -0.00015 dB/°C for a 10 mm

artifact with 0.085 dB of mean PDL and a

dependence of -0.001 dB/^C for a 17 mm artifact with 0.230 dB of mean PDL. These artifacts are

suitable both for fusion splicing as well as connectorizing the input lead.

Conclusions

We developed a nonmechanical technique of

polarization dependent transmittance measurement

for both single-mode and bulk-optic devices that offers advantages over more traditional methods.

The technique is capable of synchronous time

averaging, which allows resolution of better than

0.001 dB and agreement within 0.0016 dB ±2% of

PDL values calculated for an open-beam artifact. The accuracy can be improved in the future by

improving the accuracy of the polarization state calibration.

A new PDL artifact standard candidate is proposed. Measurements of the temperature

dependence of prototypes show PDL slopes as low

as 0.00015 dB/°C over the temperature range 0 to 40 °C.

References

[1] M. Gadonna and A. Mabrouki "Polarization

Sensitivity Measureinents Methods for Passive

Optical Components," Conference on European

Fibre Optic Communications and Networks,

EFOC&N '93, The Hague, Netherlands, Technical

Digest, pp. 65-67.

[2] R. M. Craig, S. L. Gilbert and P. D. Hale, "High

Resolution, Nonmechanical Approach to Polarization Dependent Transmission

Measurements," IEEE J. Lightwave Technol. (in press).

[3] M. A. Bhatti and A. S. Siddiqui, "A New

Polarization Dependent Loss Measurement

System," Int. J. Optoelectronics, vol. 11, no. 1, pp. 39-41, 1997.

[4] D. L. Favin, B. M. Nyman, and G. Wolter, "System

and Method for Measuring Polarization Dependent

Loss," U. S. Patent 5371597, Dec. 6 1994; B. M.

Nyman, D. L. Favin, and G. Wolter, "Automated

System for Measuring Polarization Dependent

Loss," Optical Fiber Communications Conference,

OFC '94, San Jose, California, U.S.A., Technical

Digest, pp. 230-231.

[5] Derickson, D. ed., Fiber Optic Test and

Measurement. Prentice Hall PTR, 1998, chap 9.5.

8
Measurement of Wide-Bandwidth Gain-Flat Fiber Amplifiers

Paul F. Wysocki

Lucent Technologies, Bell Labs

Specialty Fiber Devices

25 Schoolhouse Rd.

Somerset, NJ 08873

Abstract:

The impact of spectral hole burning, polarization hole burning and temperature dependent

gain on the measurement and design of wide-bandwidth EDFAs is assessed.

Summary:

Wide-bandwidth erbium-doped fiber amplifiers (EDFA) are presently in widespread

development for dense wavelength division multiplexed (DWDM) systems. To design and fully

characterize such EDFAs it is necessary to choose measurement techniques which recognize the

contributions of spectral hole-burning (SHB), polarization hole-burning (PHB) and fiber

temperature change on the resultant gain spectrum and noise figure (NF). SHB is significant in

EDFAs at room temperature, especially near 1530 nm, and it varies with signal wavelength and

EDFA compression level'" PHB is a substantially smaller effect which varies with signal

wavelength, compression and the polarization states of signals and pumps.^"^ The temperature

dependence of EDFA gain has been discussed'"^ These effects are increasingly important as gain

flatness specifications tighten to 1.5, 1.0 or even 0.5 dB total variation across 30-80 nm. EDFAs flattened using filters have recently appeared in profusion in the literature^"^ Singleband designs with 1 dB flatness over 40 nm^ and dual-band designs for about 80 nm' bandwidths

have been demonstrated. Few flat EDFAs have been measured using a full complement of

saturating signals. Until recently, acquisition of 40, 80 or 100 independently controlled signal sources has been expensive and time-consuming. However, as DWDM systems are installed,

such sources are becoming readily available. Many EDFA gain spectra in the literature have

been measured using a single or a few saturating tones to establish the erbium ion inversion and a

small-signal (either tunable or broadband) probe to measure the gain. Whether such

measurements adequately characterize the multi-wavelength gain is discussed below in light of

SHB and PHB. Typically, NF has been measured by polarization nulling or time-domain

extinction (TDE) of the saturating tone so that the ASE power can be measured. Such techniques

do not lend themselves to DWDM measurements because nulling of a large number of signals is

difficult. A computer model is used below to explore some of the design concepts and

measurement issues which are unique to producing ultra-flat EDFAs for DWDM use.

9
Spectral Hole-Burning:

Computer models of EDFAs have generally treated erbium as a homogeneous medium. Such

models are inadequate for designing filters to flatten highly-saturated EDFAs to within 1 dB.

While SHB has been measured at certain wavelengths and in certain operating conditions,' ' no

general model including SHB has been reported. The results presented here are based on a new

model which uses the measured SHB data to incorporate the correct wavelength dependence of

the SHB magnitude and hole spectral width. A portion of the EDFA saturation is assigned to each signal at each position in the fiber and SHB is added at each wavelength based upon its

contribution. The model serves as a tool for observing saturation behavior in EDFAs to obtain

precise gain and NF data. The results here are for a dual-stage EDFA made from 27.16 m of

Lucent Technologies MP980 fiber divided into a 9 m first stage and a 18.16 m second stage and

pumped with 100 mW copropagating at 980 nm in stage 1 and 150 mW at 1480 nm

counterpropagating in stage 2. This high-power in line EDFA is flattened using an interstage

filter and is assumed to include 8 dB of interstage loss to account for isolators, a variable

attenuator and other components. The total input power into the EDF is -4 dBm and the output is +21 dBm (25 dB internal gain) for all simulations.

Using a homogeneous model, a filter for the EDFA was designed, as shown in Fig. 1 . Being

an ideal filter, this produced the perfectly flat spectrum shown in Fig. 2 when modeled assuming

homogeneity. The model included 76 saturating tones spaced by 0.8 nm from 1520 to 1580 nm. When simulated with SHB, the filter was found to be improperly designed for flatness (as shown

in Fig. 2) and the gain was tilted towards longer wavelengths. Some of the gain tilt could be

removed by shortening the total fiber length (as shown in Fig.2 for 0.5 m and 1.0 m reduction)

but flatness could not be achieved. In all cases, the total output power was independent of the

presence of SHB in the model. However, the filter designed using a homogeneous model

1 520 1 525 1 530 1 535 1 540 1 545 1 550 1 555 1 560 1 565 1 570 1 575 1 580 \Afevelength (nm)

Fig. 1 : Filters for flattening 25 dB EDFA. Filter 1 is for homogeneous gain. Filter 2 accounts for SHB.

10
27 0

26.5

26.0

25.5

DO2, 25.0 c

(S 24.5

24.0

23.5

23.0

—Homogeneous Gain

Gain w/a-B

Gain w/SI-B, fiber 1 .8 %shorter — Gain w/St-B, fiber 3.6 %shorter

uLj 1520 1525 1530 1535 1540 1545 1550 1555 1560 1565 1570 1575 1580 \Afevelength (nm)

Fig. 2: Model result for filtered EDFA using Filter 1 for homogeneous EDFA and for EDFA with SHB.

Last two plots show fiber length reduction to compensate for filter inaccuracy.

produced at least 0.5 dB gain ripple, even with fiber length adjustment. Measurement of gain

using a small-signal or an out-of-band saturating tone would reveal the homogeneous spectrum,

but misrepresent the flatness achieved by the filter. A correction to the filter can be generated by

subtracting the plots with and without SHB in Fig. 2. An adjusted filter is shown in Fig. 1. The

spectrum produced using this filter is shown in Fig. 3.

It is important to consider how many saturating signals are adequate for characterizing broad

bandwidth EDFAs. The impact of SHB on EDFA measurements depends on the measurement

27.0

2&5 -

2&0 -

DDS, 25.5 I

25.0

24.5 -

-I I II 11 II II II 1 1 II 1 1 1 II 1 1 II 1 1 1 II II II I II 11 1 1 II I II 1 1 1 1 II 1 1 1 1 1 1 II 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1

24,0

Ssducbng sigrds, 64nnspabng (cbaJ 8D0Q-fe)

I I I I I I 13Sauefing Sgnals, 4.8nmspaang (SxuaDQiz)

I I I I I I I I igsatraing Sgn^s, a2rTnspaang {ixii4a)Gtv)

I I I I I I I I I I I I I I I I 38Sauc(jrp3gnE(s. 1.6rTnspacing(^iil2D0GHz)

I I I I I I I I I I I I I I I I I I I I I I I I I I leSsbJ^rg Sgnds. QSnmspadng {stal ICDQ-fe)

76Sigs

38Sigs

19Sigs

13Sigs

8Sgs

'

I 1520 1525 1530 1535 1540 1545 1550 1555 1560 1565 1570 1575 1580 Wavelength (nm)

Fig. 3: Model gain result for filtered EDFA using Filter 2 for EDFA with SHB saturated by various

numbers and spacing of channels as shown.

11
3 0 ^- I I I I .... I I 1520 1525 1530 1535 1540 1545 1550 1555 1560 1565 1570 1575 1580 VNfevelength (nm)

Fig. 4: Model NF result for filtered EDFA using Filter 2 for conditions of Fig. 3.

technique and the EDFA design. Any technique which fully loads the EDFA and accurately measures the gain and generated ASE versus wavelength is ideal. Techniques which substitute

less channels must meet two conditions to be accurate. The first condition requires that the

reduced number of channels saturate the EDFA to the same level produced when all channels are

present. In fact, if the measurement is to characterize the NF properly, it must reasonably

recreate not only the average ion inversion but also the distribution of ion inversion. The second

condition is that, in any wavelength range, the saturating signals must not be spaced substantially

as far apart as the SHB hole width, unless this is the full EDFA loading condition.

Fig. 3 shows the spectrum of an EDFA predicted by model for a variety of different channel

spacings and wavelengths in which the total power was distributed equally amongst the

saturating tones. It shows that a 100 GHz channel spacing can be replaced by a 200 GHz spacing

without noticeable difference (condition 2 is not violated). A 400 GHz spacing is an adequate

substitute for signal wavelengths above 1538 nm but not near 1530 nm where SHB is a strong

effect (condition 2 violated near 1530 nm). The total output power was virtually independent of

the choice of channels. This is expected to be the case only for highly-saturated, high-power

EDFAs, but not for small-signal preamplifiers or low power in-line EDFAs. The NF of this

EDFA is shown in Fig. 4. Interestingly, it was virtually independent of the choice of saturating

tones, as long as the total input power was distributed uniformly across the band.

Condition 1 above is usually achieved by distributing the substitute channels to guarantee

nearly the same distribution of saturating power with wavelength. Condition 2 is the tougher

condition to meet and requires far more channels. Even if condition 2 is violated however, both

power output and NF measurements may still be accurate for highly-saturated EDFAs as long as condition 1 is still met. Substitution of a single saturating tone has often been used. This was

12
27

26 -

CQ2. 25 cO

24 -

23

Fully Loaded EDFA

1550 nm Channel, -0.85 dB

1550 nm Channel, -0.69 dB

1550 nm Channel, -0.53 dB

1550 nm Channel, -0.38 dB

1550 nm Channel, -0.23 dB

1550 nm Channel, -0.09 dB

1550 nm Channel, Full Power

1520 1530 1540 1550 1560 VNfevelength (nm)

1570 1580

Fig. 5: Single 1550 nm channel saturation result for EDFA at various power levels,

simulated here using a 1550 nm saturating tone, very near the center of the band. This clearly

violates condition 2 but may or may not violate condition 1. The spectrum for various signal

levels is shown in Fig. 5. Because this saturating tone does not approximate the full EDFA load,

the gain at full power is low. When the signal power is reduced, the average gain can approach

the fully loaded case, but the spectrum is seriously distorted. In fact, far from 1550 nm, the error

is just the difference between the two filters of Fig. 1. At these wavelengths, the homogeneous

gain has been measured, not the fully loaded gain.

Polarization Hole-Burning and Polarization Dependent Loss:

Fortunately, polarization-dependent loss (PDL) is usually a small effect in a modem WDM

test set because available components have low PDL. However, PDL of some EDFA

components is still an issue. PHB has been extensively studied for single-channel EDFAs.^"*

Signal-induced PHB increases the orthogonal ASE in a measurement, which tends to makes NF

measurements high. However, since no single channel does much of the overall saturating in a DWDM EDFA, the effect is small. The likelihood of any substantial number of channels being in

like polarization state for a substantial portion of the EDFA is small. The effect of PHB from all other channels on a given charmel is on average 0. The main PHB effect in a DWDM EDFA is expected to be pump-induced PHB. This again averages to 0 but a coincidental alignment of a pump and a single channel can change its gain be several tenths of a dB.^ Of course, a 980 nm

pump is expected to wander in polarization state relative to a 1550 nm signal, so its PHB effect

should be reduced. A 1480 nm pump may maintain polarization relative to a signal for longer.

Scrambling of the pump polarization and averaging all measurements over several scrambling

cycles can eliminate this effect.

13
26.0 r

25.5

EDF Designed for 25 dB Gain

Flattened at 20C from 1 528-63 nm

Rots for constant average gain

^ 25.0 - c

03 o

24.5 -

24.01520 1530 1540 1550 1560 V\/&velength (nm)

1570 1580

Fig. 6: Gain spectrum of typical EDFA as a function of temperature.

Temperature Dependent Gain:

Tight tolerances on gain flatness have forced EDFA designers to consider the temperature

dependence of the EDF gain. To model the dependence, the absorption and gain coefficients of a

typical high aluminum EDF have been measured as a function of temperature. These coefficients were used to predict the change of gain expected in an EDFA with temperature. For a 25 dB

EDFA flattened at 20 C, the predicted change in gain spectrum with temperature is shown in Fig.

6. The gain ripple produced by a 20 C change is on the order of 0.5 dB. The implication is that a

flat DWDM EDFA must be designed and measured at its nominal operating temperature and

across all temperatures of use. To guarantee excellent flatness over all operating conditions,

temperature control of the entire EDFA may be needed.

References

1. J. Sulhoff, A. Srivastava, C. Wolf, Y. Sun and J. Zyskind, IEEE Photonics Technology Letters, Vol. 9,

p. 1578 (Dec. 1997).

2. A. Srivastava, J. Zyskind, J. Sulhoff, J. Evankow and M. Mills, Proc. Optical Fiber Conf. OFC 96,

San Jose, CA, p. 33 (1996).

3. P. Wysocki and V. Mazurczyk, IEEE Journal ofLightwave Technology, Vol 14, p. 572 (Apr. 1996).

4. V. J. Mazurczyk and J. L. Zyskind, IEEE Photon. Technol. Lett., Vol. 6, p. 616 (1994).

5. J. Kemtchou, M. Duhamel, F. Chatton, T. Georges and P. Lecoy, Proc. Optical Amplifiers and their

Applications '96, Paper FD2, p. 126 (1996).

6. J. Lee and N. Park, Proc. Optical Fiber Conf. OFC 98, San Jose, CA, paper WGl (1998).

7. P. Wysocki, Proc. Optical Fiber Conf OFC 98 San Jose, CA, paper (1998).

8. P. Wysocki, J. Judkins, R. Espindola, M. Andrejco, A. Vengsarkar, and K. Walker, Tech. Dig.

OFC '97, paper PD2 (1997).

9. Y. Sun, J. Sulhoff, A. Srivastava, J. Zyskind, C. Wolf, T. Strasser, J. Pedrazzani, J. Judkins, R. Espindola and A. Vengsarkar, Tech. Dig. Optical Amplifiers and Their Applications 97, paper PD2.

14
Polarisation Mode Dispersion

in Chirped Fibre Bragg Gratings

David Ives National Physical Laboratory

Teddington, Middx, TWll OLW, UK

Abstract

An explanation of the high level of polarisation mode dispersion, PMD, in chirped fibre Bragg gratings

is reported. Initial measurements support the theory and suggest that PMD in such gratings could be a

significant contribution to the total PMD in a fibre system.

Introduction

Chirped Bragg gratings written into optical fibres are now readily available for use as chromatic

dispersion compensators[l][2]. Such devices are only beneficial if they accurately compensate

chromatic dispersion without impairing other system parameters. PMD is an important parameter in long haul high bit rate systems. Initial measurements of PMD in chirped fibre Bragg gratings are reported here.

Theory

Consider an ideal linear chirped grating. Consecutive wavelengths are reflected at progressively further or nearer positions along the grating giving a time delay which varies linearly with wavelength (a constant chromatic dispersion). The wavelength, x, 's reflected at a point z along the grating given by

z = aX+ b

where a and b are constants. This leads to a time delay, i. 2n„ 2n

T = -Z% ^ - 6 (aX+b)

c c where n^ is the group index and c the speed of light. The chromatic dispersion, D, is given by

D = ^ =

dX c The grating is formed from periodic refractive index variations with a spacing, S, given by

S = A = ilb

2n 2na

where n is the average refractive index.

For real fibre there will be some residual birefringence. The refractive index for light propagating

parallel to the birefringent axis and perpendicular to the birefringent axis is then given by

P ^ P

nil - n-- and n± = n+-

where, p, is the birefringence.

Thus, at a given wavelength, light propagating parallel to the birefringent axis is reflected at the point,

Z|, where x\ "S given by

15
while light propagating perpendicular to the birefringent axis is reflected at the point, Zi, where Xi is given by

This difference in reflected position leads to a corresponding difference in the propagation times for the

parallel and perpendicular modes giving a differential group delay (DGD), Ax,

Ax = —^ ^ = DX^

c n n Assuming that the birefringence is independent of wavelength (such that the birefringence and group

birefringence, Pg, are approximately equal) the DGD coefficient. A, for light propagating straight

through the grating is given by

c c Hence the DGD for the straight through and reflected signals can be related through

Ax = DXAn

It can be seen that the DGD for the reflected signal is a large multiple of the DGD for the straight

through signal.

PMD Measurements

The set up used to measure PMD at NFL is shown in Figure 1. Radiation from the tuneable laser source passes through a polarisation rotator before being launched into the grating under test. The

output polarisation state from the grating is measured using a polarisation state analyser.

Tunable

Laser

Source

A

System

Controller

Chirped Fibre

Bragg Grating

nnn

Polarisatioh

Rotator

A

Circulator

1

^

3

Thermal Enclosure

Polarisation ^—

: State

;Analyser / /

/"^

Figure 1 PMD Measurement System

The output polarisation state is measured for a number of different input states [typically having

16
normalised stokes vectors (1,0,0), (0,1,0) and (0,0,1)] at each wavelength step. Prior to making

measurements the polarisation state controller settings are adjusted for each wavelength to launch the

required state. The measured data is analysed by calculating the rotation of the stokes vector on the

surface of the Poincar^ sphere[3][4]. This gives values of the DGD and the principle states of

polarisation (PSP)[5] for each wavelength step. Polarisation dependent loss and second order PMD can

also be estimated.

Measurements

Test were performed on a dispersion compensating grating with a centre wavelength of 1549.2 nm and

a bandwidth of 1 nm. The chromatic dispersion was measured to be approximately 700 ps.nm"'. The

grating was used with an optical circulator to provide a transmission dispersion compensation system.

The PMD of each element was measured. Measurements were made in the wavelength range 1484 nm

to 1564 nm at 20 nm intervals. Six launched polarisation states were used with normalised stokes vector (1,0,0), (0,1,0), (0,0,1), (-1,0,0), (0,-1,0) and (0,0,-1). This gave a differential group delay

sensitivity of approximately 1 fs.

The optical circulator had a DGD of 47 ± 5 fs from port 1 to 2 and 49 + 5 fs from port 2 to 3. By

applying a 4% reflection to port 2 the PMD of the combined route 1 to 2 and 2 to 3 could be

measured. PMD values consistent with combinations of the single pass values were obtained.

The optical fibre grating was measured in transmission giving a DGD value of 22 + 3 fs. All of these

measurements were performed outside the reflective region of the grating.

Where the uncertainties quoted correspond to a 95% confidence interval.

The grating and circulator were combined to achieve dispersion compensation. Measurements of the

output polarisation state were measured in the wavelength range 1548.6 nm to 1549.7 nm at 0.1 nm

intervals. Three launched polarisation states were used with normalised stokes vectors (1,0,0), (0,1,0)

and (0,0,1). In this geometry a DGD of 6940 ± 1250 fs was obtained. The principle state of

polarisation was observed to have only a slight wavelength dependence across the measurement range.

Comparison with theory

The measured forward DGD of the fibre grating can be used to calculate the birefringence of the fibre

in which the grating is fabricated. The total connectorised grating length of 3.1 m with its DGD of 22

fs results in an average DGD coefficient of 7.1 fs.m"'. This in turn results in an expected PMD for the

grating in reflection of 1540 fs. This compares well with the measured value.

While it is expected that there will be no mode coupling within the length of the grating there may be some mode coupling in the pigtail fibre at each end of the grating. This will increase the apparent

DGD coefficient of a short piece within the pigtailed length. A PMD coefficient of ^ fs.m ' will result

in a total PMD of

At = aVHi"

where I is the fibre length and h the coupling length[6]. For the grating measured, taking h = 0.15 m

and so ^ = 32 fs.m ' and the PMD for the grating in reflection would be calculated at 6940 fs, as measured.

17
The PMD contributed by the optical circulator is not considered to be significant.

Fine Structure

The output polarisation state of the compensation system was monitored as a function of wavelength. It was observed that the rate of rotation of the stokes vector around the principle axis varied with

wavelength. This suggests a variation of the DGD. This variation occurred over a wavelength period

of approximately 0.05 nm and was observed through the whole wavelength scan. The magnitude of the

periodicity of these changes compares well with those observed in the time delay curve. Accurate

measurement of the DGD for small variations of wavelength proved difficult because of the noise

induced in the DGD associated with the wavelength step accuracy.

Conclusion

A simple theory to explain the high values of DGD within chirped fibre Bragg gratings has been

developed. Initial measurements support the theory but structure on a small wavelength scale needs

further investigation. The grating tested had a chromatic dispersion of approximately 700 ps.nm ' and a PMD of 6940 fs. While the chromatic dispersion will compensate for perhaps 40 km of standard

optical fibre the PMD will add to the system total giving an extra PMD which will appear as 1 ps.km'

This is larger than the typical fibre specification of 0.5 ps.km "'^

Acknowledgement

This work was supported by the DTI National Measurement System Policy Unit.

References

1) HILL K.O., BILODEAU F., MALO B., KITAGAWA T., THERIAULT S., JOHNSON D.C.,

ALBERT J., TAKIGUCHI K., "Chirped in-Fiber Bragg Gratings for Compensation of OpticalFiber Dispersion", Optics Lett., 1994, 19, ppl314-1316.

2) LOH W.H., LAMING R.I., ELLIS A.D., ATKINSON D., " 10 Gb/s Transmission over 700 km of

Standard Single-Mode Fiber with 10 cm Chirped Fiber Grating Compensator and Duobinary

Transmitter", IEEE Photonics Tech. Lett., 1996, 8, ppl258-1260.

3) ANDRESCIANI D., CURTI F., MATERA F., DAINO B. "Measurement of the Group-Delay

Difference between the Principle States of Polarization on a Low-Birefringence Terrestrial Fibre

Cable", Optics Utt., 1987, 12, pp844-846.

4) POOLE CD., BERGANO N.S., WAGNER R.E., SCHULTE H.J. "Polarization Dispersion and

Principle States in a 147km Undersea Ligthwave Cable", 7. ofLight Tech., 1988, 6, ppll85-1190.

5) POOLE CD., WAGNER R.E. "Phenomenological Approach to Polarisation Dispersion in Long

Single-Mode Fibres", Elect Lett., 1986, 22, ppl029-1030.

6) GISIN N., VON DER WEID J. P., PELLAUX J. P. "Polarisation Mode Dispersion of Short and

Long Single-Mode Fibres", 7. ofLight Tech., 1991, 9, pp821-827.

18
Measurement of Polarisation Mode Dispersion

in Fibre Amplifiers

C. Geiser, B. Huttner, N. Gisin (1);

R. Caponi, M. Potenza, M. Schiano, M. Artiglia (2);

I. Joindot (3)

(1) GAP, Universite de Geneve, Geneve, SWITZERLAND (bruno.huttner@physics.unige.ch)

(2) CSELT, Torino, ITALY (marcello.potenza@cselt.it)

(3) France Telecom/CNET, Lannion, FRANCE (irene.joindot@cnet.francetelecom.fr)

Abstract: The behaviour of the Polarisation Mode Dispersion (PMD) in various types of

optical fibre amplifiers has been studied by the European consortium FAME (Fibre Amplifier

Measurement Methods). We present some guidelines for precise measurement of PMD in amplifiers. Our main result is that the PMD of such devices is inherently deterministic and

can be specified simply using a maximum tolerable value.

Introduction

The Standard, Measurement & Testing European project FAME (Fibre Amplifier Measurement MEthods) is involved

in the characterisation of Optical Fibre Amplifiers for advanced applications such as multi-channel and analogue

transmission systems. One of the major issues of FAME is to get a satisfactory operative definition of Polarisation Mode Dispersion (PMD) for such amplifiers. The problem, which could present some difficulty, originates from the fact that the usual way to measure the PMD on a fibre, consists in averaging the Differential Group Delay (DGD) over some

wavelength interval and is based on the assumption that PMD in a fibre is statistical in character /I/. Amplifiers usually

contain elements (filters, isolators, wavelength selective couplers...) which restrict DGD measurements to short wavelength intervals. Moreover, the gain spectrum of OFAs further limits this region to a typical operation interval of

1530-1570 nm for Erbium-Doped Fibre Amplifiers (EDFAs) or Erbium-Doped Fluoride Fibre Amplifiers (EDFFAs)

and of 1270-1330 nm for Praseodymium-Doped Fluoride Fibre Amplifiers (PDFFAs). Furthermore, in three-level OFAs PMD must be measured under pumping conditions suited to assure a significant Degree Of Polarisation (DOP)

level during the measurement.

This contribution reports on the experimental work carried out by the FAME Consortium. We give some guidelines for precise measurement of PMD, in particular with respect to pump and signal powers. We show that the DGD of the amplifier is very stable, even on a scale of weeks and in presence of large environment temperature fluctuations. This

indicates that a proper definition for PMD in OFAs should be given a deterministic character. This result is of

considerable relevance for standardisation and customer/contract specifications of OFAs.

Guidelines for PMD measurements

A set-up for PMD measurements is shown in Fig. 1. A tunable source launches the optical signal - the power of which

is maintained in the -13 to-3 dBm range - into a State-Of-Polarisation (SOP) generator (polariser). The output of the SOP generator is injected in the Amplifier Under Test (AUT). The device output is sent to a polarimeter through an

optical attenuator, to avoid detector saturation. The polarisation behaviour of the attenuator is checked preliminarily, to verify that it cannot affect results significantly.

The PMD calculation is performed with the Jones Matrix Eigen-analysis. Amplifiers under test were four conventional

EDFAs, four EDFFAs and one PDFFA, including commercial line- and booster amplifiers, and laboratory prototypes.

19
Fig. 1 - Set-up for the measurement of PMD in OFAs

GPIB interface tunable source

mainframe polariser polarimeter

optical attenuator

A first measurement on an EDFA was made with the pump off, and shown in Fig. 2. The DGD is drawn as a full curve, and presents an unexpected and somewhat irregular behaviour. In the region between 1510 and 1540 nm, we see that DGD is extremely low and is not representative of the real DGD of the amplifier. Our explanation is that in this wavelength region, the radiation at 1550 nm is absorbed and incoherently re-emitted by Er-atoms. Since the re-emitted

light is not polarised, the DOP drops (as seen on the dashed curve of Fig. 2), which makes the measurement unreliable. This behaviour is observable only because the attenuation is so high (30 dB), that the intensity of incoherently re- emitted light is roughly the same as that of directly transmitted light. We can simply avoid this behaviour by turning the pump on, thus turning attenuation into gain.

In addition, when the pump is on, the unpolarized ASE may perturb the measurement. In order to obtain significant

results, the probe power must also be large enough, so that the output signal is larger than the ASE. As above, this can be checked by measuring the DOP. We choose the input power so that the DOP remains between 60% and 100%.

The principle of the Jones Matrix Eigen-analysis used for the amplifier's PMD measurement imposes another condition,

especially when the pump is turned on. The measurement consists in analysing successively three linear output states of

polarisation (SOP). Imagine the worst case, when the SOP of the probe is orthogonal to one of the three polarizers. The

probe power is then set to zero and we analyse only unpolarized ASE instead of the expected amplified signal. The

solution to avoid this problem is simply to choose a circular SOP, so that the probe intensity after any of the three polarizers is the same.

Fig. 2 - DGD measurement on an EDFA: unpumped conditions

1470 1480 1490 1500 1510 1520 1530 1540 1550 1560 1570 Wavelength [nm]

20
To conclude this section, we state again the three conditions necessary for a correct measurement of PMD with the standard polarimetric method:

• Pump on (gain)

• Probe intensity large enough, i.e. signal laser larger than ASE in order to get DOP > 60 %

• Circular input polarisation before the polarising stage

Results of PMD measurements

In Fig. 3, we present a set of measurements performed on a commercial EDFFA.

Fig. 3 PMD of a cofnmercial EDFFA;

DGD curves measured at different power levels and different measurement settings

0.16 -

Q 0.08 - o

° 0.06 -

0.04 ~

0.02 -

0 . . . 1530 1535 1540 1545 1550 1555 1560 1565 1570 Wavelength [nm]

We see that, once the conditions discussed above are met, the DGD of the device as a function of wavelength remains

largely independent of the measurement set-up. The time dependence of DGD was also analysed, for one EDFA. Fig. 4 shows 89 measurements of the PMD, taken every 30 minutes during 2 days in stable conditions. It is manifest that each curve is quite the same as all others except

for little details. The DGD seems to be stable in such conditions. We also made longer-term measurements, by

comparing measurements made at an interval of one month. The result was that even after such a long time, the DGD at a fixed wavelength remained stable. Lastly, we measured the PMD of the amplifier at different temperatures. The result was again the same curve, but slightly shifted in ft^equency. This can simply be explained by the thermal variation of the Er-doped fibre length or of other EDFA components.

Our conclusion is that the PMD of the EDFAs we measured is a deterministic PMD, which is not affected by random

coupling changing with time or with external conditions, but results from the intrinsic birefringence and stable polarisation mode couplings.

Fig. 4: Stability of DGD curves measured every half an hour along two days

1.1 ,,

0,6 I o

1 1525 1530 1535 1540 1545 1550 1555 1560

^ [nm]

21
Conclusion

In this work, we have pointed out some difficulties encountered when measuring the PMD of an ampHfier. First, the pump must be turned on, to avoid depolarisation effects. Second, for a given pump power, the probe power has to be

larger than ASE. These two conditions can be checked by measuring the DOP of the output signal before any PMD

measurement. We found that a minimum value of 60% was appropriate. Third, the input SOP has to be circular before passing through the linear polarizers in the polarimeter. This precaution allows us to obtain the same probe power for the three independent SOPs when measuring the Jones matrix of the AUT.

We have tested the stability of DGD with time and found that it was excellent. We have also analysed its temperature

dependence, and found it consistent with a PMD created by intrinsic birefringence. Our conclusion is thus that the PMD

of the Er-based amplifier is deterministic. Therefore, it is sufficient to measure the DGD in the window of interest. Such

a deeper understanding of the PMD nature is a key feature for a standardisation of polarisation mode dispersion tailored

to optical fibre amplifier characteristics (in contrast to the usual definition for transmission fibres). Besides, it is also important for device customer and contract specifications. Further work is under way to assess the PMD nature in different (still non-conventional) OFAs, such as PDFFAs for the second telecommunication window and others, in which possible imperfections of the fibre structure could play a role in adding a partially stochastic component to the device PMD.

Acknowledgements

This activity has been partially funded in the framework of the European SM&T Project FAME and by the Swiss Office Federal de I'Education et de la Science.

References

/I/ N. Gisin, B. Gisin, J. P. Von der Weid, R. Passy, "How accurately can one measure a statistical quantity like polarisation mode dispersion?", SOFM'96, Boulder (CO), Tech. Dig. p. 131 (1996), or in IEEE Photon. Technol.

Lett., Vol. 8, No. 12, pp. 1661-ff , ( 1996).

121 N. M. O'Sullivan, T. A. Birks, C. D. Hussey, "Control of polarisation degradation in fibre amplifier WDMs",

Electron. Lett. Vol. 28, No. 17, pp. 1616-1618, (1992).

131 Y. Namihira, T. Kawazawa, H. Taga, "Polarisation effects on BER degradation at 10 Gbit/s in an IM-DD, 1520 km

optical amplifier system", Electron. Lett. Vol. 29, No. 18, pp. 1654-1655, (1993).

/4/ R. Leners, T. Georges, P.L. Fran(jois, G. Stephan, "Analytic model of polarisation dependent gain in erbium doped

fibre amplifiers", Conf on Optical Amplifiers and Their Applications, 1994, Yokohama (Japan); Tech. Dig. Post- deadline papers.

22
Narrowband measurements of polarization-mode

dispersion using the modulation phase shift technique

p. A. Williams', A.J. Barlow^, C. Mackechnie\ and J.B. Schlager'

'National Institute ofStandards and Technology, Boulder CO, USA

'EG&G Fiber Optics, Wokingham, UK

A common characteristic of the current methods for measuring polarization-mode

dispersion (PMD) is the need for a broad optical bandwidth. With the increasing use of

wavelength division multiplexing the need to make PMD measurements within a very narrow optical bandpass will also increase. One technique which is well suited to narrowband measurements of PMD is the modulation phase shift technique (MPS). While

the concept is not new [1-4], we believe the work here to be the first published

demonstration of the accuracy of the technique in measuring differential group delay

(DGD) vs. wavelength.

MZ Modulator

Tunable

Laser

InGaAs

Detector

Polarization

Controller H2-JL

Spliced PMF

Test Device Narrow Band Filter @ 1.920 GHz

Vector

Voltmeter

Figure 1 Schematic diagram of modulation phase shift system for DGD measurement.

The MPS technique is well known for its use in chromatic dispersion measurements [5]. The basic principle is to directly measure the group delay of light traveling through each

of the two principal states of polarization of the device under test. The experimental setup

is shown in Figure 1. The light fi-om a tunable laser diode with an RF-broadened

linewidth of 50-500 MHz undergoes an external 1.920 GHz intensity modulation irom a LiNbOs Mach-Zender modulator. The modulated light passes through a polarization

controller, passes through the device under test, and is detected with a high-speed InGaAs

photodetector and filtered with a narrowband filter at 1.920 GHz. A vector voltmeter is used to measure the phase between the RF electrical modulation signal and the optical

signal out of the device under test. The phase is a measurement of the group delay

through the optical system, so adjusting the polarization controller to maximize the

optical phase delay means that the light is being launched down the slow principal axis of

the device under test. Likewise, the polarization orientation for minimum phase delay

corresponds to a launch down the fast principal axis. The difference in phase between

these two launch conditions is a measure of the DGD at the wavelength under test. That

23
is, DGD (At) is given by

Ar (p^ -(p.

(1)

360°-/

where At is in seconds, (p+ and cp. are the measured phases (in degrees) for the

transmission along the slow and fast axes respectively, and/= 1.920x10^ Hz.

In order to demonstrate the accuracy of this technique, we made a measurement on three

concatenated segments of bow-tie polarization-maintaining fiber (PMF) oriented at roughly 45° angles so as to mix polarization modes, providing a DGD with a strong

wavelength dependence. The maximum DGD of the PMF concatenation is about 8 ps. The vector voltmeter used in this measurement has a phase resolution of 0.1° which at 1.920 GHz corresponds to a time resolution of 0.145 ps. A 360° phase change would

represent a 520 ps delay. Therefore, given the PMD of the device being measured, there

is no danger of a In phase ambiguity. Using the MPS technique, we measured the DGD

of the PMF device over about a 20 nm range and compare the results with DGD-vs.-

wavelength measured by the more conventional Jones matrix eigenanalysis (JME)

method [6]. Measurement results for the two techniques are compared in Figure 2.

10

P

O

6

4

2

0

If'

.30,

1495 1500 1505 1510 1515 1520 1525 1530 1535

Wavelength (nm)

Figure 2 DGD vs Wavelength for JME technique (dots) and MPS technique (diamonds) on a three-section PM fiber.

It should be noted that the JME measurements were made at a temperature ~3 °C lower

than the MPS measurements. The temperature dependence of the birefringence (zlw) of

PMF d(zl«)/dr is on the order of -4xlO"^/°C [7]. This means that for a single section of

PMF, a few degrees of temperature change will affect the DGD by only a few tenths of

one percent. However, a change in the birefringence of the fiber sections also changes the

mode-coupling conditions. This effect depends on the retardance of the sections, which is proportional to AnL/?i (where L is the length of the PMF section and A is the optical

wavelength). From this relationship, we see that a change in AnL will be equivalent to a change in X (wavelength shift). Multiple measurements of the PMF test device using the

24
JME system showed an effective wavelength shift with temperature of -0.8 nm/°C. We

therefore anticipate that the data taken at the lower temperature (JME) would be offset by

about 2-3 nm. The agreement between the JME and MPS data is good when we shift the

JME data down by 2.5 nm. This value gives the best fit and corresponds well to a 3 °C

temperature shift. Consequently, the JME data shown in Figure 2 have been shifted by

2.5 nm.

The uncertainty in the MPS measurement comes primarily fi-om two sources: phase

uncertainty due to the resolution of our vector voltmeter and the degree to which we were

able to launch exactly on the principal axes. Fabry-Perot effects in the measurement

system are significantly reduced by RF-broadening the source, and the residual random

phase uncertainty is completely explained by the 0.1° phase uncertainty of the vector

voltmeter. The uncertainty due to launching nonprincipal states is a source of systematic

error. The measured phase difference between two orthogonal launch states is a ftanction

of the alignment of the launch states with respect to the principal axes. Figure 3 shows

the fi-actional DGD error encountered for a given alignment error. Alignment error is the

angle (on the Poincare sphere) between the launch polarization state and the slow

principal axis of the device under test. Any misalignment error therefore causes a reduction in the measured DGD. Fortunately, the error is a weak fianction of

misalignment for small angular errors. For example, a 5° misalignment angle will yield a measured DGD which is only 0.4% below the true value.

We found the principal state by merely manipulating the

polarization controller by

hand and watching for the

maximum and minimum

phase delays. Repeated

measurements showed that

the measured phase

differences for repeated runs

differed by no more than 0.2°

(consistent with the phase

resolution of the system).

Figure 2 shows that the JME

and MPS techniques agree

within the measurement

noise. Therefore, our

technique of finding fast and

slow axes by hand appears to be accurate (but tedious).

In order to reduce measurement noise and add some degree of automation, we are experimenting with a differential phase (DP) measurement technique [1]. For the DP

measurement, we use a commercial chromatic dispersion test system and an

electronically controlled three-stage polarization controller. The first stage (a half-wave

-0.0001

0 15 30 45 60 75 90

Misalignment Angle (deg)

Figure 3 Fractional DGD measurement error as a

function of launch state orientation (0° is ideal launch

along principal states, 90° is worst-case misalignment).

25
plate) is used to toggle between orthogonal states of polarization at a rate of tens of Hertz.

The other two stages are used to search for the principal states of the device under test. Preliminary results predict an improved DGD resolution with a much narrower 200 MHz

bandwidth requirement.

References

[1] Roger S. Jones, "Fiber optic dispersion method and apparatus" United States Patent

4,740,833 (1985).

[2] M. Monerie, P. Lamouler, L. Jeunhomme, "Polarisation mode dispersion

measurements in long single mode fibers", Electronics Letters 16, 907-908 (1980).

[3] S.E. Mechels, Measurements ofthe zero-dispersion wavelength in single-mode

opticalfibers, Ph.D. thesis. University of Colorado (1996).

[4] Dennis Derickson, Ed., Fiber optic test and measurement, p. 512, Prentice Hall, New

Jersey (1998).

[5] Anonymous, "Chromatic dispersion measurement of single-mode optical fibers by the

phase-shift method". Fiber Optic Test Procedure (FOTP) 1 69, Telecommunications

Industry Association.

[6] B.L. Heffner, "Automated measurement of polarization mode dispersion using Jones

matrix eigenanalysis", Photonics Technology Letters 4, 1066-1069 (1992).

[7] A. Ourmazd, Malcohn P. Varnham, R.D. Birch, and David N. Payne, "Thermal

properties of highly birefringent optical fibers and preforms". Applied Optics 22, 2374-

2379(1983).

26
Impact of signal wavelength and input state-of-polarization on PMD induced

pulse distortion in >10 Gbit/s transmission systems

Ralph Leppki, Arnold Mattheiis'

^University of Kaiserslantern, Erwin Schrddinger Strafie, D-67633 Kaiserslautern

^Deutsche Telekom AG, Technologiezentrum, Am Kavalleriesand 3, D-64295 Darmstadt

Abstract: We present experimental results and numerical calculations which show

the dependence of pulse distortion caused by polarization mode dispersion (PMD)

on both the differential group delay (DGD) at the signal wavelength and the state-ofpolarization (SOP) of the light launched into the fiber. The maximum pulse

broadening versus to the input SOP is related to the DGD at the signal wavelength.

The results demonstrate the possibility to achieve nearly undistored transmission at any wavelength by proper adjustment of the input SOP. Conclusions are drawn for

>10 Gbit/s data transmission in single channel and in WDM systems.

1. Introduction

In the last few years PMD has attracted considerable attention due to its potential limitation on optical transparent high speed transmission systems. PMD can cause system penalties [1] or even detrimental signal distortions and thus unacceptable bit error rates [2, 4] when installing

the next generation systems (STM-64 / OC-192) on already embedded cables. It is often argued

that the impact of PMD on pulse propagation along a fiber piece cannot be predicted in a

deterministic way because of the stochastic nature of PMD. This is not entirely true. In previous

long-term PMD measurements on installed standard monomode fibers (SMF) [2, 4] it was

observed that the DGD and the output SOP usually remain stable for at least several minutes for

any input SOP and signal wavelength. In this case it seems to be justified to approximate the

transmission fiber by a concatenation of birefringent elements (CBE) with well-defined angles

for the fast and slow axis of each birefringent element.

In this article we present a simple experimental setup of a CBE which was used to compare

theoretical and experimental results for both, firstly the DGD versus wavelength and input SOP,

and secondly the impact of PMD on pulse distortion. Recent > 10 Gbit/s field trials are mentioned confirming the applicability of this simple model of a CBE for optically transparent

systems on installed fibers.

2. Experimental setup

The experimental setup is shown in Fig. 1. A wavelength-tunable mode-locked fiber-ring

laser generated Fourier-limited nearly sech^-shaped pulses (FWHM) of about At,n=6 ps and

70 GHz temporal and spectral width (FWHM) at around 1550nm central wavelength,

respectively. The pulseform is adapted to a 40 Gbit/s transmission system using return to zero

27
format. These pulses passed a polarization control unit (PCU) - enabling to manipulate the input

SOP - before being launched into a CBE, namely a chain of ten identical pieces of polarization

maintaining single-mode fiber (PMP^. Each PMF segment had a geometrical length of 1 m and

offered a wavelength-independent DGD of i = 1.5 Ips. The input SOP behind the PCU and the

spectral and temporal output pulse shapes were measured by a polarization analyzer based on the

Jones-Matrix-Eigenanalysis (JME) method, an optical spectrum analyzer and a streak camera,

respectively.

' BireMngent Fiber

Streak Camera

Fig. 1: Experimental setup for measurement of PMD induced pulse-distortion versus wavelength and input-SOP

3. Theory

In our numerical calculations each PMF piece was described by a delay matrix D while the

interface between two subsequent PMD pieces was represented by a rotation matrix R according

to well-established previous approaches (see e.g. [5]). Thus the Jones-Matrix J(A.) of the CBE

was defined for each wavelength X. The DGD and PSOP were found using the PolarizationDispersion-Matrix J(^i)- J~^(/^2) [3]- The output pulse was calculated by multiplying each

Fourier component of the pulse with its corresponding Jones-Matrix.

4. Results

Firstly, all coupling angles between the slow and fast axis of subsequent PMF pieces were

calibrated by measuring the DGD versus wavelength X and minimizing the standard deviation

28
between the experimental and simulated DGD(^). A global minimum and thus a well-defined set of angles was found.

Next the output pulse shape was measured versus the input SOP by rotating the polarization

analyzer's half-wave plate. Thus the output pulse shape was tuned from the undistorted pulse

(which was identical to the input pulse) up to maximum distorted pulse (showing two separated

peaks). An example is illustrated in Fig. 2 foe X=\541.S nm central wavelength. As can be seen

the output pulse width Atout covered the range of 6 ps < Atoui ^ 12.5 ps. In Fig. 3 the maximum

distorted pulse of Fig. 2 was depicted along with the corresponding numerical simulation.

Similar good agreement between measured and simulated output pulse shapes was also observed

for other signal wavelengths.

80 1 00

time [ps

Fig. 2 Pulse-distortion for different input SOP at a

tlxed signal wavelength (/^sianai=1547.8 nm)

Fig.

10 20

time [ps]

30 40

3 Comparison of numerical calculation

and measurement for the maximum

distorted output pulse.

simulation measurement

</) a

O

Q5

-

1530

Fig. 4

1535 1540 wavelength [nm] 1545

DGD at center-signal / \ wavelength / \

I

\

input SOP

Measurement and calculation of DGD Fig. 5 Measured output pulse width (FWHM)

of CBE. for different input SOP at different

signal wavelengths.

29
Both, measured and simulated data revealed that the DGD versus wavelength is a periodic

function in sequences of AX=X"/c/r = 5.3 nm with t = 1.51 ps (Fig. 4) resulting from the periodic

structure of the CBE. Ne.xt the impact of the input SOP on the output pulse width was analyzed

for various signal wavelengths (Fig. 5). Similar to the particular example presented in Fig. 2 the

input SOP was tuned along a circle of the Poincare sphere. The maximum output pulse width was correlated to the wavelength (and the corresponding DGD). The minimum output pulse

width was approximately the same (namely At„m=At,n=6ps) for all wavelengths.

5. Summary and Conclusion

It was shown that PMD induced pulse distortion at each signal wavelength can be

manipulated between a maximum and a minimum value by changing the SOP of the light

launched into the fiber. Thus an almost undistorted pulse transmission can be reahzed even if the DGD exceeds the pulse width. The maximum achievable pulse distortion is correlated to the

DGD at the signal wavelength. This wavelength dependence may yield different signal

degradation for the various channels of a WDM system.

We observed that the polarization behaviour of installed single-mode fibers is pretty well

reflected by the deterministic model of CBE discussed in this paper while commonly used

stochastic models are too pessimistic. The DGD and the PSOP of optical transparent high speed

systems in the field remain stable for at least several minutes, and the input SOP can always be

optimized for almost undistorted signal transmission [1, 4]. In a recent 10-Gbit/s field trial [1]

with more than 40 ps PMD the BER varied from 10"^<BER<10"'" without polarization control. It was possible to stabilize the system at the minimum BER just by controlling of the input SOP in

sequences of minutes.

References

[1] Niall Robinson, Gary Davis, John Fee, 4 x SONET OC-192 Field Installed Dispersion

Managed Soliton System over 450 km of Standard Fiber in the 1550 nm Erbium Band,

OFC'98, Postdeadline Paper, San Jose, 1998.

[2] Ralph Leppla, A. Mattheus, J. Ruffler, F. Wittl, H. Haunstein, B. Teichmann, L.C. Garcia,

Polarization induced system penalties in 10 Gbit/s field trial at 1,3 pm using

semiconductor optical amplifiers, COST workshop on SOA-based Components for

Optical Networks, Prague, October 27-28, 1997.

[3] Bemhard Scholl, Faseroptischer Polarizationsanalysator, Shaker Verlag ISBN

3-8265-1522-6, 1996.

[4] H. Billow, G. Veith, Temporal dynamics of error-rate degradation induced by

polarisation mode dispersion fluctuation of a field fiber link, Proc. ECOC'97, Vol. 1, pp.

1 15-1 18, Edingburgh, September 22-25, 1997.

[5] N. Gisin, R. Passy, J. P. Von der Weid, Definitions and measurements of polarization

mode dispersion: Interferometric versus fixed analyzer methods, IEEE Photonics

Technology Letters, vol. 6, pp. 730-732, 1990.

30
Repeated Measurement of Polarisation Mode Dispersion

in Installed Cables

Adam KAPOVITS

PKI Telecommunications Development Institute,

MATAV - Hungarian Telecommunications Company Ltd.

Abstract: Nowadays the effect and importance of polarisation mode dispersion (PMD) is well known and measurement instruments are commercially available. Repeated

measurement ofPMD in installed cables is reported in this paper. The PMD offibres in two

different cables were measured on three different occasions. The measurements were performed with commercially available, but different measurement systems. PMD was found to vary significantly with time. Measurement results are discussed and some trends are identified. However, no explanation is foundfor the large changes.

Introduction

In the last few years much attention has been paid to polarisation mode dispersion (PMD). This is due to the effect of PMD on transmission quality. Therefore in case of a telecommunications network operator it

is essential to characterise its cable network in this respect. In this paper we report on repeated

measurements taken on long installed cable sections with different commercially available measurement

systems from two different manufacturers.

Discussion

PMD was repeatedly measured on the dark fibres of two relatively long cable sections, the 47 km long

Budapest-Bicske section, and the 57 km long Budapest-Dabas section. Both cable sections comprised of

underground cables, but of different construction and from different suppliers. The fibres in the two cable

sections were also different. In case of the Budapest-Bicske section the fibres were manufactured with the

OVD (Outer Vapour Deposition) method, while in case of the Budapest-Dabas section the fibres were

manufactured with the MCVD (Modified Chemical Vapour Deposition) method. The cables were installed about 7 and 6 years ago, respectively.

PMD at 1550 nm was measured Dn three occasions, in May 1996, March 1997 and November 1997. PMD

at 1310 nm was measured in May 1996 and November 1997, only. The measurement instruments were

kindly lent by two different manufacturers. Both commercially available instruments are using the

interferometric technique with broadband sources to measure the PMD. (In May 1996 and November 1997

the measurements were taken with different versions of the same instrument from the same manufacturer.)

We have found that both equipment are well suited for field use and a number of measurements could be

taken in a short time.

In some cases fibres were re-measured on the same day (after a few minutes or a few hours) or on the

second day.

31
The PMD delay values of the Budapest-Dabas section measured at 1550 nm are presented in Figure 1, while PMD delays of the Budapest-Bicske section (also measured at 1550 nm) are presented in Figure 2. At 1550 nm the fibres of the Budapest-Dabas section have moderate PMD coefficients (<0,4 ps/yjkm ) , while the fibres in the cable of the Budapest-Bicske section have low PMD coefficient (<0,2 ps/^Jkm ), so they are excellent in this respect. Actually this might be the reason for the large fluctuation of the measured

PMD.

3

2,5

a 2

1

0,5

0

Budapest-Dabas section, PMD @ 1550 nm

I

A

t j

ft

Meas. no.l. 16/05/96 Meas. no.2. 16/05/96 A 17/05/96 Meas. no.l. 07/11/97

A Meas. no.2. 07/ni/97

OMeas. no.3. 07/11/97

i

i

> *

*

8 10

Fibre No.

12 14 16 18 20

Figure 1.

1,4

1,2 a 1 O 0,8

Z

t 0,6 s

0,2

0

Budapest-Bicske section, PMD @ 1550 nm

10 15 Fibre Ho.

-^

<

] Meas. no.l. 16/05/96 Meas. no.2. 16/05/96 A 1 7/05/96 X 28/03^7

i1

s

! d

06/11^ 7

20 25 30

Figure 2.

32
In Figure 3 and 4 the PMD delays of the fibres of the Budapest-Dabas and Budapest-Bicske section

(measured at 1310 nm) are presented, respectively.

Budapest-Dabas section, PMD @ 1310 nm

1

i

1

> i E . Ar—- - . a I B

1

i t

> Me< Me<

is. no.l. 1

IS. no.2. 1

6/05/96 6/05y96 _

7/11/97

A17/35y96

Meas. no.1. 0 AMeas. no.2. 0 7/11/97

0 2 4 6 8 10 12 14 16 18 20

Fibre No.

Figure 3.

Budapest-Bicske section, PMD @ 1310 nm

Q 0,8 n

ir

i

I 1 17/0!;/96 06/11/97

0 5 10 15 20 25 30

Fibre No.

Figure 4.

33
Key Findings

• The measurements showed that the fibres have good, sometimes very good PMD performance. It was

so, despite the fact that PMD was not covered by the specifications when the cables were procured. The

tight specifications acted indirectly and ensured good PMD performance.

• The PMD values measured at 1310 and 1550 nm are in the same order, but there is no tendency of

being smaller (larger) at one of the wavelengths.

• There are cases, when the PMD values can vary significantly from measurement to measurement, from

one day to another, as it is in case of fibre 5, 15 and 28 of the Budapest-Bicske section at 1550 nm (see

Figure 2) or fibre 19 and 20 of the Budapest-Dabas section at 1310 nm (see Figure 3). Probably the

best example for such a behaviour is, however, fibre 1 of the Budapest-Dabas section at 1550 nm (see

Figure 1). • There are cases (e.g. fibre 1 9 and 20 of the Budapest-Dabas section) when large short term variation in

the PMD at 1310 nm could be observed, while the PMD at 1550 nm is fairly stable.

• The two measurements taken in 1997 on the Budapest-Bicske section at 1550 nm were in a good

agreement, except in case of fibre 5 (see Figure 2).

• There is a clear tendency, that PMD measurement results in 1997 are lower than those obtained in 1996. This holds for both cable sections and for both wavelengths. There are cases when the PMD

values measured in 1997 are almost the same as those measured in 1996 (i.e. sometimes the difference

is less significant), but the 1 997 measurement results are always lower than the 1 996 ones.

Conclusions

In some cases large variations could be observed from measurement to measurement. The repeatability of

the PMD measurement was found relatively poor, especially from 1996 to 1997. This finding is especially

difficult to explain, since the cables are in the ground, in stable conditions. Until now we do not know the reason for that, but the scatter of the measured PMD values was significantly larger than the theoretical

limit [1].

References

[1] N. Gisin, B. Gisin, J. P. Von der Weid, R. Passy, "How Accurately Can One Measure a Statistical Quantity Like Polarization-Mode Dispersion?", IEEE Photonics Technology Letters, Vol. 8., No.

12., December 1996, pp. 1671 -1673

34
Twin-photon techniques for fiber measurements

N. Gisin J. Brendel and H. Zbinden

Group of Applied Physics, University of Geneva, 1211 Geneva 4, Switzerland

A. Sergienko and A. Muller

Dept. of Electrical and Computer Engineering, Boston University

July 7, 1998

1 Introduction

Characterization of single-mode optical fibers requires a basis for time, wavelength and polarization.

Parametric downconversion in non-linear crystals naturally provide pairs of photons extremely

highly correlated in time, energy (thus wavelength) and polarization. It is thus tempting to explore

such photon pairs for fiber and fiber device characterization. Historically, entangled photon pairs

where first used in delicate tests of quantum mechanics [1]. Indeed, their correlation is higher than

classically possible! However, nowaday photon pair sources can be made cheap and compact enough

to offer practical alternatives for many of the traditional measurement schemes. In addition photon

pairs provide entirely new possibilities and open the door to new developments in metrology. The

purpose of this contribution is first to present some concrete proposals and results along the above

lines, and next to draw the attention of the audience to these rather revolutionary new combinations

of quantum optics and communication.

2 A photon pair source

Figure 1 presents an example of a photon pair source. A non-linear crystal, like BBC, KNbO^,

LiI02, is pumped by a 15 mW laser diode, at 660 nm in the present example. The prism between

the laser diode and the crystal removes all extra infra-red light. The crystals are well known for fre- quency doubling, as in doubled Nd-YAG lasers. Here we use the time reversed process: wavelength

doubling, better known as downconversion (recall that all elementary quantum processes are time

symmetric). Since initially the infrared modes are empty (ie are in their quantum vacuum state and the power of pump radiation is not very strong), this downconversion process is spontaneous:

a photon from the pump spontaneously splits into two infrared photons. Energy and momentum

conservation forces these two photons to be highly correlated: they are created exactly at the same

time and the sum of their energy (optical frequency) equals the energy of the pump photon. Note

that the latter is defined within the spectral width of the laser, typically expressed in MHz, hence

the sum of the two downconverted photon is well defined. But the spectral width of each downconverted photon can be quite large, up to tens of nm, or even hundreds of nm. Following the

optical path on Fig. 1, the photon pairs are then injected into a single mode fiber, with standard

techniques (microscope objective in the shown example). Next, a directional coupler separates both

photons and guides them to two optical connectors. Figure 1 presents one possible implementation. Other examples are discussed in the next sections. For instance, it might be useful to replace the directional coupler by a wavelength demulti35
Figure 1: Example of a compact twin-photon source.

plexer: the two photon are still quantum correlated (ie extremely highly correlated) in time and

energy, but the photon at one output is in the short wavelength end of the spectrum, while the

companion photon is in the complementary long wavelength range. In fig. 1 a collinear configuration is present: the pump and both downconverted photons follow parallel trajectories. Another

possibility exploits momentum conservation: if one photon emerges at an angle with respect to the

pump beam, the companion photon emerges on the opposite angle (the detail depend on the exact

phase matching conditions, wavelength filters, etc). This will be illustrated in the next section for various dispersion measurements.

So far we did not mention polarization, but since the crystals are anisotropic, polarization is important (actually birefringence is required for the phase matching condition). There are two

kinds of downconversion processes. In type I both infrared photons have identical polarization,

both orthogonal to the pump laser's polarization. In type II downconversion, on the contrary, the

two infrared photons can be totally unpolarized, however their polarization states are quantum cor- related: if one passes through a polarizer, the other one is immediately polarized in the orthogonal

direction. This will be illustrated in subsection 4.3 for polarization mode dispersion measurement.

Other examples of interesting configurations for 2-photon sources are described in [2, 3].

3 Photon counters

Photomultipliers exist for years, but are not eff"ective at telecom wavelengths (1.3 and 1.55 /j). Photon counting devices based on silicon Avalanche Photo-Diodes (APD) are commercially available, but also limited to wavelengths below 1 /i. However, a few groups around the world have

demonstrated that germanium and InGaAs APD are suitable for photon counting at telecom wavelengths [4]. Admittedly, this is still the weak point of "quantum optics at telecom wavelengths",

because most of the demonstrations used liquid nitrogen cooled APD. Actually, very recently we

have demonstrated that InGaAs APDs Peltier cooled are suitable for metrology applications where

noise requirement is not as severe as for quantum communication applications. Anyway, progress

in this field is fast. The fact that until now not much efi'orts have been put into these developments,

allows one to be optimistic about a possible breakthrough in the near future.

36
" Two-Photon Source. x= 1320.8 nm Wavelength Resolved 2-Photon Coincidence Detection

Figure 2: Setup for chromatic dispersion measurements.

4 Dispersion measurements

The intrinsic time-energy-polarization correlation of the downconverted photons make them natural

candidates for dispersion measurements, as illustrated in the next 3 subsections.

4.1 Chromatic dispersion measurements on long fibers

This is possibly the simplest example. It exploits only the time and energy correlations of the

photons: the difference in detection times provides the information on the time of flight and the

measurement of the wavelength of one photon is enough to obtain the necessary information on the

wavelength of the companion photon. Therefore we have direct access to the group delay between

complementary wavelengths. Choosing moreover the central wavelength near the zero chromatic

dispersion, these delays will be small, thus not necessitating any long electronic delay lines. In

short, this technique makes use of the photon pairs as a broadband lightsource, comparable to a LED, with sub-picosecond pulsewidth.

A possible implementation is presented in Fig. 2. For technical details we refer the interested

reader to [5]. In our first experiment we could determine the zero chromatic dispersion wavelength

with a precision of ±0.5 nm [5]. In a forthcoming experiment, we expect to achieve higher resolution,

larger spectral width (up to 300 nm) and measurement times of minutes.

4.2 Chromatic dispersion measurements on short fibers

The spectra of the downconverted photons are large, like LEDs or even broader. However, the

photon pairs, considered as a whole, still enjoy the high coherence of the pump laser. Hence, using

2-photon interferometry, the chromatic dispersion of meters long samples of fiber can be determined.

This is similar to the well-known white light interferometric measurement technique [6], but with

the advantage that the fiber sample length does not need to be adjusted with millimeters accuracy.

Thanks to the meter long coherence of the pump laser, the sample length needs only to be adjusted

within cm or meter precision. The proposed setup is presented in Fig. 3. Admittedly, this example is more complex than the previous one. Depending on the temperature of the crystal the wavelengths

of the collinearly emitted photons vary between 1250 and 1350 nm for one photon and between

1450 and 1600 nm for the other, the central wavelength being at 1400 nm. The interferometer is unbalanced, having the test-fiber in the long arm and a wavelength division multiplexer (WDM)

and a moveable mirror in the short arm. The two photon have four choices: The first one can either

take the long arm and the other one the short one or vice versa, or both photons take either the

short or the long arm. The first two cases are distinguishable due to the time difference between

37
Jh—I Ar+ Laser

BBO crystal Type-ll Sample

Variable polarization delay line ~

7e~r'ay7v~ l

I ——nvl

ARD , Detector 2

. ' i o-ray LI

AFD

Detector 1

Analyzer

Analyzer

Figure 4: Setup for PMD mesurements.

the arrival times of the two photons. The latter two possibilities are indistinguishable, hence lead

to two photon interference. The visibility of the interference pattern is reduced, if the dispersion

is not identical in the two arms of the interferometer. We can compensate for the difference in

the group velocities in test fiber between the photon around 1300 nm and the photon around 1500

by separating them with the WDM in the short arm and by introducing an adjustable delay for one of them by a moveable mirror. The position of this mirror for optimal interference visibility

is a measure for the time delay between the two wavelengths introduced by the dispersion in the

test fiber. We can now scan over the whole wavelength range by tuning the temperature of the

nonlinear crystal and obtain the dispersion between 1250 and 1600 nm, thus obtain the group delay

as function of wavelength.

4.3 Polarization mode dispersion measurements

In this example, the photon pairs are produced by type II parametric downconversion. Hence, in

addition to their time and energy correlations, they are also entangled in polarization, thus suitable

38
Optical Delay (fs)

Figure 5: Result of the PMD measurement with sub-femtosecond resolution.

for Polarization Mode Dispersion (PMD) measurements. A possible implementation is presented in

Fig. 4, for details see ref. [7]. In this experiment, birefringent plates where actually measured, not

fibers, and the source was an Argon laser. However, the results clearly demonstrate the potential

of the method, see Fig. 5. Essentially the method is similar to the white light interferometric

technique, but thanks to the wide spectrum of the downconverted photons (about 300 nm is this

experiment), a resolution of 0.01 fs was achieved. This should be contrasted with the 50 to 100 fs resolution of the standard interferometric technique based on LEDs [8].

5 Absolute calibration of sources and detectors without blackbody radiation

As a last example, we present the use of photon pairs for the determination of the absolute efficiency

of detectors. Let us emphasize that this technique, contrary to all other techniques we are aware

of, is not based on a light source calibrated with black body radiation. Hence, this new technique

allows to determine detector efficiencies independently of temperature. The basic idea is simple [9]. Since the downconverted photons are always produced by pairs, the number Nab of coincidence

detections with two detector A and B equals:

Nab = NrjAVB (1)

where N is the total number of photon pairs, and ija and r)B are the efficiencies of detectors A and

B, respectively. Moreover the single counts at detector B equals Nb = NrjB- Consequently, the

efficiency of detector A is given by the simply relation:

Nab

This elegant idea has now been tested in several labs [9], in particular at the NIST-Washington,

and the accuracy limitations due to the statistics of photon counting has been estimated [10]. A

trial to extend this technique to analog detectors is presented in [11]. Analogoulsy, a calibrated source can be devised.

6 Conclusion

The potential of highly correlated photon pairs produced in parametric downconversion for optical

fiber and devices has been emphasized. The huge technological progress in simple and efficient

39
photon pair sources and the ongoing progress in photon counting techniques at telecom wavelengths

open new possibilities for the fiber optic community. The intrinsic correlations of the photon

pairs offer advantages such as automatic determination of a common time basis, complementary

wavelengths and polarization. Thus all the degrees of freedom of photons in single-mode fibers are entangled in these photon pairs. Each photon may be totally depolarized and cover a bandwidth

of tens or even hundreds of nanometers. Now, at the same time the two photons form a single

quantum object (a bi-photon) with large coherence lengths, up to meters. This offers the possibility to combine advantages of low and high coherence interferometry.

The examples presented in this contribution are but a few ones, more should be expected. Finally, let us briefly mention that quantum optics has also interesting potentials for communication.

As examples we mention quantum cryptography [12] and noiseless amplifiers that beat the 3dB

"quantum limit" [13].

Acknowledgments

This work profited from support by the Swiss Priority Program in Optics and by the European

TMR network on " The physics of quantum information"

References

[1] see e.g. J. Freedman, and J. F. Clauser, Phys. Rev. Lett., 28,938-941, (1972); A. Aspect, P. Grangier,

and G. Roger, Phys. Rev. Lett., 47, 460-463, (1981); Z. Y. Ou and L. Mandel, Phys. Rev. Lett., 61,

50-53, (1988); Shih and Alley, Phys. Rev. Lett. 61, 2921, (1988); P. R. Tapster, J. G. Rarity, and P. C. M. Owens, Phys. Rev. Lett., 73, 1923-1926, (1994). W. Tittel et al., Phys. Rev. A 57, 3229, 1998.

[2] P. G. Kwiat et al., Phys. Rev. Lett., 75, 4337, (1995). ??? nb400

[3] P. Kwiat, CLEO/IQEC, pp, San-Fransisco, 1998.

[4] F. Zappa et al. Optics Lett. 19,846-848,1994; P.C.M. Owens et al. Applied Optics 33,6895-01,1994;

G.L. Morgan et al, Los Alamos National Laboratoty report LA-UR-97-4375, 1997; G. Ribordy et al. Applied Optics, in press, 1998.

[5] J. Brendel, H. Zbinden and N. Gisin, Optics Commun., in press, 1998.

[6] L. Thvenaz et al, J. Lightwave Tech. 6, 1-7, 1988.

[7] A.V. Sergienko et al., preprint, Boston University, 1998.

[8] N. Gisin et al, JEOS Pure & Applied Optics 4, 511, 1995.

[9] see e.g. A.V. Sergienko and A. N. Renin, Applied Optics, 30, 3582, 1991; A.L. Migdall et al., Metrologia

32, 479-483, 1995; G. Brida et al, preprint, Istituto Electtrotecnico Nazionale G. Ferraris, Torino, Italy,

1998.

[10] P. Kwait et al. Applied Optics 33, 1844, 1994; N. Gisin, J. Brendel and A. Stefanov, preprint, Geneva

University, 1998.

[11] A. Sergienko and A.N. Renin, Sov. Tech. Phys. Lett. 12, 328, 1986.

[12] W. Tittel, G. Ribordy and N. Gisin, Phys. World, March 1998.

[13] D.J. Lovering et al, Optics Lett. 21, 1439, 1996.

40
COMPARISON BETWEEN INTERFEROMETRIC AND POLARIMETRIC

PMD MEASUREMENT TECHNIQUES

BASED ON FIBER IMPULSE RESPONSE

Up to now, the assumption of a gaussian impulse response for fibers affected by Polarization Mode Dispersion has

represented the theoretical background of the interferometric measurement technique. This paper introduces a new

model, based on the theory of the Principal States of Polarization, which leads to an original interpretation of the interferometric results and establishes a clear relationship between the Interferometric and the Polarimetric PMD

measurement methods. Experimental data supporting the new model are also presented.

1. Introduction

Comparison between Interferometric (IM) and Polarimetric (PM) PMD measurement methods has represented an

important issue since the early development of the two techniques [1, 2, 3]. However, the time and wavelength

random fluctuations of PMD and the deep difference between the theoretical approaches of the two measurement

methods made this task quite difficult. In particular, the fiber Impulse Response (IR) due to PMD is a fundamental element in the IM technique theory [4] that models the fiber as a concatenation of linearly birefringent waveplates with random orientations. On the contrary, to the author's knowledge, the IR has never been calculated on the basis of the Principal States of

Polarization (PSPs) theory [5], where the PM techniques are established [6, 7]. In paragraph 2 it is demonstrated

that, according to the PSPs theory, the IR has a two-lobe shape and, in general, is different from the gaussian

hypothesis of [4]. The calculation of the IR starting from PM measurements allows a direct comparison with IM

measurements, not only in terms of average Differential Group Delay (DGD) of the PSPs, but also in terms of full interferometric pattern. This is the subject of paragraph 3 that illustrates the new comparison technique by means of

examples based on experimental data. Although this comparison technique is somewhat complex and is still sensitive to PMD time fluctuations, it represents a well-established relationship between IM and PM methods and offers the opportunity of further investigations on the spectral behavior of the PSPs.

2. Impulse response evaluation

The impulse response of a fiber affected only by PMD is of course a theoretical abstraction: a broadband optical

input pulse, indeed, can not be considered unaffected by chromatic dispersion. Nevertheless, it becomes an

important tool when IM measurements are considered. In an IM set-up [4], the interferometer measures the autocorrelation function, I(T), of the fiber output field component, linearly polarized along a given direction, when

the input is a broadband, incoherent, CW light source:

E(T) is the electric field component parallel to the interferometer polarizer (or to the polarization eigenaxis in case of

a polarization maintaining fiber interferometer).

Although the modulus of the power spectrum of the light source is similar to that of a short pulse source, however its phase is a random function rather than zero and thus E(T) can not be regarded as the fiber IR. Nevertheless, when it passes through the interferometer, the random phase difference between E(T) and E(T+t) is averaged by the photodetector and the resulting interferogram can be considered an approximation of the IR autocorrelation function.

Marco Schiano

CSELT, via Reiss Romoli 274, 10148 Torino (Italy)

Abstract

41
Furthermore, interference occurs only between spectral components at the same wavelength traveling with the same

group velocity. This makes chromatic dispersion negligible in IM measurements. To conclude these preliminary considerations, it should be reminded that PMD produces a random modification on a

signal travelling in a fiber and therefore IR must be determined on a statistical basis. In the following, the approach

of [4] is adopted: IR is calculated as the Probability Density Function (PDF) of the Group Delay (GD) of the electric

field, regardless of its input polarization state. As already stated, a fiber affected by PMD only is considered and polarization dependent loss and chromatic

dispersion in the whole bandwidth of interest are neglected.

According to the PSPs theory, the group delay of a signal traveling on the slow PSP is given by:

-t„+At/2,

and similarly for a signal traveling on the fast PSP:

71 = - At / 2 , where T,„ and At are the mean group delay and the DGD of a given couple of PSPs respectively.

If a statistical ensemble of PSPs is considered. At must be regarded as a random variable and hence and T. , as

linear functions of At, became random variables as well. T„, has been implicitly assumed constant because, as far as

it concerns PMD measurements, neither IM nor PM techniques are sensitive to the mean GD of PSPs, but only to their DGD.

It is now straightforward to derive the probability density functions of 7+ and T from that of ziT[8]:

/, (T; ) = 2g[2{T, - Tj] , fAT) = 2g[-2{T - rj]

,

where j is the PDF of Zlrand the factor 2 is a normalization factor.

If the input signal is unpolarized, then both slow and fast PSPs can be considered equally excited and the PDF of the group delay of such a signal is simply the normalized sum off+(T+) and f.(T.): f(T) = g[2(T - T„, )] + g[-2(T - TJ] , (1)

where T is the group delay regardless of the PSP.

The function /frj consists of two replicas of the PDF ^f- ), symmetric with respect to T,„, with a time scale factor of

2. Some examples of measured /(Tj are shown in the next paragraph. It should be noted also that the effectiveness of

relation (1) is not limited to the case of random DGD, but it also fits the case of polarization maintaining fibers, where the PDF of the DGD is a Dirac function.

Finally, it is important to remind that, in the IM set-up, the interferometer selects a linear polarization state aligned

with a given direction. Applying the total probability theorem [8] it is easy to demonstrate that equation (1) modifies

in: f{T,X)^4x g[2{T - TJ] + VT^g[-2{T - TJ] , (2)

where X is the fraction of optical power coupled from a slow PSP to the linear polarization state of the

interferometer eigenaxis: + COS??), J? being the angle between the unit Stokes vectors representing the

two polarization states [9]. This second random variable X, which is itself a function of time and wavelength, makes the impulse response (2), and hence its autocorrelation function, more complicated and susceptible to time fluctuations.

3. Comparison of interferometric and polarimetric techniques

Experimental evidence of the previous model has been searched by PMD measurements on an ITU-T G.652 fiber

plant, 72 km long, installed in buried ducts. IM measurements have been carried out with an unpolarized LED

source, whose 3 dB bandwidth ranged from 1480 nm to 1570 nm. PM measurements have been performed by the Jones Matrix Eigenanalysis (JME) method [6], in the same spectral range, with a wavelength step of 1 nm. JME

measurements have been repeated several times in order to collect a number of samples large enough for PDF

calculation. Then the PDF of the JME data set was determined, and by means of equation (1) the IR of the fiber was

calculated as well. At the end, the autocorrelation function of the calculated IR was compared with the IM

measurement. This procedure is illustrated in detail in figure 1, while other examples of the final comparison are reported in figure 2.

42
differential group delay PDF impulse response

time [ps]

impulse response autocorrelation

0.02

100

0

T3 50 • "5. £

CO

0 -

<D T3

"5. E

CO

time [ps]

results comparison

1"

10

f

5

'.• rlA^

n ^ ^ \V

-2 -1 0 1 2 time [ps]

Figure l-Comparison procedure for IM and PM methods: the picture labeled results comparison reports the autocorrelation function of the IR calculated from PM data (doted line), compared to the measured

interferometric pattern (continuous line)

results comparison results comparison

Figure 2-Further comparison of IM patterns measured at different times (continuous line) with the

autocorrelation function of the calculated IR (doted line)

43
Some important observations can be derived from these experimental results. First of all, it should be noted that the central peak of the interferogram is the autocorrelation function of the source spectrum and is not related to PMD [3], so the mismatch of the compared functions around zero is not significant. A

second remark is that the autocorrelation function of the IR calculated from PM data, i. e. the autocorrelation of (1), does not take into account fluctuations of the PSP's Stokes vector direction. For this reason, measured

interferometric patterns, which on the contrary depend on the random variable X, are more irregular than the calculated autocorrelation function of the IR, and present wide time fluctuations. Unfortunately, up to now,

commercially available polarimeters can not measure this quantity which could improve the comparison accuracy. Furthermore, due to small misalignment of the interferometer, interferometric patterns are slightly asymmetric and

this degrades the comparison quality as well. Taking into account previous considerations, the results of the comparison technique can be considered satisfactory

and well supporting the theoretical model.

3. Conclusions

A new comparison technique between interferometric and polarimetric PMD measurement methods has been

presented. It is based on the calculation of the impulse response of a fiber affected by PMD according to the theory

of the Principal States of Polarization, and allows a direct comparison of the interferometric pattern with the autocorrelation function of the impulse response calculated from Polarimetric measurements. Experimental results indicate that, despite time fluctuations of interferometric patterns, a quite satisfactory agreement between the two

methods can be found. A further improvement of the technique could be achieved by the measurement of the wavelength fluctuations of the Stokes vectors representing the output PSPs, that is, at present, not allowed by

commercial polarimeters. Finally, a revision in the data analysis algorithms of interferometric patterns seems to be

advisable to fit the behavior of experimental data that are, at the same time, non gaussian-like and time fluctuating.

Acknowledgments

The author gratefully thanks Dr. T. Tambosso for continuous encouragement and Dr. P. Morra for preparing the

plant.

References

[1] N. Gisin et al., Experimental comparison between two different methods for measuring polarisation mode

dispersion in singlemode fibres, Electronics Letters, Vol. 27, No. 24, pp. 2292-2293, November 1991.

[2] N. Gisin, B. Perny, F. Prieto, C. Zimmer, Polarisation Mode Dispersion: large scale comparison ofJones

Matrix Eigenanalysis against Interferometric measurement techniques, Electronics Letters, Vol. 32, No. 7, pp.680-681, March 1996.

[3] B. L. Heffner, Influence of optical source characteristics on the measurements of polarization-mode dispersion

of highly mode-coupledfibers. Optics Letters, Vol. 21, No. 2, pp.1 13-115, January 1996.

[4] N. Gisin, J. P. Von der Weid and J. P. Pellaux, Polarization mode dispersion of short and long single-mode

fibers, Journal of Lightwave Technology, Vol. 9, No. 7, pp. 821-827, July 1991.

[5] C. D. Poole, R. E. Wagner, Phenomenological approach to polarization dispersion in long single-mode fibres. Electronics Letters, Vol. 22, No. 19, pp. 1029-1030, 1 1* September 1986.

[6] B. L. Heffner, Automated measurement ofpolarization mode dispersion using Jones matrix eigenanalysis,

Photonics Technology Letters, Vol. 4, No. 9, pp. 1066-1069, September 1992.

[7] D. Andresciani et al.. Measurement of the group-delay difference between the principal states ofpolarization on

a low-birefringence terrestrial fiber cable. Optics Letters, Vol. 12, No. 10, pp. 844-846, October 1987.

[8] A. Papoulis, Probability, Random Variables and Stochastic Processes, McGraw-Hill, New York, 1991.

[9] CD. Poole, D.L. Favin, Polarization-mode dispersion measurements based on transmission spectra through a

polarizer. Journal of Lightwave Technology, Vol. 12, No. 6, pp. 917-929, June 1994.

44
Determination of Small PMD Values via the Poincare Sphere Method

Using Wavelength Scanning Data

N.Cyr

EXFO Electro-Optical Engineering, Inc. 465, av. Godin, Vanier, Quebec, Canada, GIM 307

1. Introduction

Polarimetric PMD-measurement techniques, such as the Jones Matrix Eigenanalysis' (JME) and Poincare

Sphere^ (PS) methods, permit the PMD to be fully characterized by determining the optical frequency

dependence of the polarization dispersion vector Q. The magnitude of this vector is the differential group

delay (DOD), and its orientation yields the principal states of the device under test (DUT). The PMD is defined as the mean or rms value of the DOD averaged over all optical frequencies. In the case of the PS

technique, the output state of polarization (SOP) describes a curve on the Poincare sphere, whose local angular velocity yields the DOD. Both the JME and PS methods require the measurement of the three

Stokes parameters describing the SOP as a function of optical frequency, a frequently tedious acquisition

process which can take many minutes to carry out using a tunable laser and a polarimeter. Other com- monly used approaches to PMD measurement, such as the interferometric^ (INT) and wavelengthscanning fixed analyzer^ (WSFA) methods, measure the projection of the output SOP along one axis characteristic of the measurement instrument, for a rapid determination of the PMD. However, the information

lost in 'discarding' the other two components severely hampers measurement of extremely small (<50-fs)

PMD values, particularly when the medium exhibits intermediate or strong mode coupling.

We describe here a method permitting a PS-determination of the PMD using 'incomplete' polarimetric

data. The measurement of small values of PMD using this PS variant requires only two 'polarimetric' pa- rameters, which can be deduced from WS measurements with different linear axes of analysis. This timesaving technique is based on the fact that it is the magnitude and not the sense of the local angular velocity on the Poincare sphere which determines the DOD.

2. Measurement concept

Fig. 2 shows a schematic diagram of the measurement concept. The output from a broadband light source (BBS) is polarized by a linear polarizer (PI). The light exiting the DUT is analyzed with another linear

polarizer (P2). An optical spectrum analyzer registers the optical-frequency-dependent transmission

through P2. The setup is similar to that of broadband wavelength scanning^ except that polarizer PI and

analyzer P2 can be rotated. A number of spectra are registered and used in combination to obtain trajectories of the output SOP on the Poincare sphere.

In the simplest implementation, one needs to perform three spectral acquisitions: successive WS measurements with the analyzer P2 oriented at 0, 45, 90°, (and, optionally as a crosscheck, 135°), the sum of

the powers Pq + Pgg (or P45 + P135) being equivalent to a measurement of the spectrum at the input of P2.

On account of this normalization of the transmission, the spectral shape of the broadband source is not

important. In reality, this implementation is overly simplified. In order to ensure that the input SOP is not by chance aligned with one of the principal states of polarization (PSP) of the DUT, the above-mentioned

45
\ Rotatable

Polarizers

Figure 2:. Schematic diagram of the measurement concept. The polarizer PI is oriented at 0 or 45°, and

the analyzing polarizer P2 at 0, 45, 90 or 135°.

procedure must, in general, be repeated with the first polarizer PI rotated by 45° from its inital position.

From these measurements, two of the three normalized Stokes parameters, i.e. Sj and S2 , can be calculated

in terms of Pq , P45 and Pqq (all as a function of optical frequency v). Moreover, since the light at a given

optical frequency is fully polarized, we can also deduce S3 to within a sign factor:

s,(v)

2Pn(v)

Po(v) + P9o(v)

s,(v) =

2P,5(v)

Po(v) + P9o(v)

s.(v) s?(v) + s5(v) (1)

Given the trajectories of the output SOP on the Poincare sphere, PMD can be deduced in different ways.

Two main categories of situations lead to different analysis of the raw data.

In the weak mode-coupling regime, the trajectory on the sphere is an arc of a circle, its projection in the

equatorial plane (S3 = 0) is an ellipse, and the Si(v) are cosine functions with a period 1/DGD. We can make use of this property to obtain the DGD by fitting data points with cosines. Residual mode coupling

or non-zero differential chromatic dispersion induce a slight change in both orientation and magnitude of

the polarization dispersion vector with optical frequency (second-order PMD). An accurate value of the mean DGD is obtained by fitting cosines with varying periods, amplitudes and offsets.

In the strong mode-coupling regime, a piecewise analysis of the trajectories must be undertaken. Using

the two trajectories in combination, the Poincare sphere method^ can be applied to obtain the DGD for each small interval of optical frequency. Although a mathematical ambiguity appears where the trajectory crosses the equatorial plane (S3 = 0) (since we know only the magnitude of S3 and not its sign) this is nor- mally resolved by considering the trajectories corresponding to each of the two signs and verifying which

of the two represents a smooth continuation of the curve before the zero crossing.

On the other hand, a simpler procedure consists in fitting cosines piecewise to obtain the mean DGD for each interval of optical frequency. The procedure do not require a knowledge of S3; only the projection of

the trajectories onto the equatorial plane is needed. In order to remain practical, this analysis should be

restricted to small PMD values (< 0.3 ps). For large PMD values (> 1 ps), the trajectory would need to be

divided into an excessively large number of intervals.

Of course, the small PMD technique described here could include an explicit measurement of S3 by means

of a supplemental spectral acquisition whereby a quarter waveplate was placed in front of P2 in Fig. 2, at the expense of the overall measurement time and complexity.

46
3. Experiment

The technique is first illustrated by the measurement of a very small PMD in weak mode coupling. The

DUT is a segment (7.5 cm) of intermediate-birefringence fiber (200 fs/m), pigtailed with short lengths of

standard single-mode fibers. The broadband source is the superposition of two LEDs (1310 and 1550 nm),

with a small but usable power in between. Figure 3 shows spectra registered by an OSA for one orientation

180 215 250 180 215 250

Optical frequency ITHz] Optical frequency [THz]

Figure 3: Observed spectra, (a) Analyzer P2 at 0° (~) and 90° (-—). (b) at 45° (-^ and 135° (-—).

T 1

-1 ^10 1

180 215 250

Optical Frequency [THz]

Figure 4: Measured trajectory and cosinefit, mean DGD = 14.3 fs. (a) Stokes parameters s^ and Sj as a

function ofopticalfrequency, (b) Projection ofthe trajectory onto the equatorial plane.

of PI, with four settings (0, 45, 90, 135°) of the analyzer orientation. Stokes parameters s,(v) and S2(v),

obtained from the spectra according to Eq. (1), are plotted in Fig. 4(a) as a function of optical frequency,

along with cosine fits as described in section 2. The two curves are superimposed. The mean DGD is 14.3

fs. Preliminary results indicate the repeatability of such measurements to be within 0.1 fs.

The second example is a 3 km long spool of SMF fiber. The observed spectra are shown in Fig. 5. The

Stokes parameters S](v) and S2(v) deduced from these spectra are plotted in Fig. 6(a) as a function of optical frequency. The projection of the trajectory in the equatorial plane, shown in Fig 6(b), is no longer an

ellipse. In such a case, a single cosine function no longer suffices to fit each of Si(v) and S2(v) across the

full frequency range. Instead, the projections are fitted piecewise with cosine functions to obtain the mean DGD in each interval of optical frequency (see section 2.). The average over the whole frequency range

obtained this way is 68 fs.

47
c

o

Oh

12

o

A

1 \

' \

180 210

Optical frequency [THz]

240 180 210

Optical frequency [THz]

240

Figure 5: 3 km ofspooledfiber: observed spectra, (a) Analyzer P2 at 0° (—-) and 90° (—-). (b) at 45°

0

I

/ ^

1

I 1 ^\

V. V

180 210

Optical Frequency [THz]

240

Figure 6: Measured trajectory with the 3 km spooledfiber, (a) Stokes parameters 5j and Sj as afunction

ofopticalfrequency, (b) Projection of the trajectory onto the equatorial plane. Mean DGD = 68fs.

4. Conclusions

We have shiown tliat a frequency-domain polarimetric analysis can be made, for an accurate determination

of subpicosecond PPvlDs, from a limited number of WS measurements. The technique described here can be carried out in a much shorter time than standard polarimetric PMD methods. Moreover, it can be ren- dered even more rapid if used in conjunction with an 'adaptive' algorithm to limit the measurement to only

two standard WS acquisitions if, as may often be the case, the DUT exhibits weak mode coupling.

References

1. B.L. Heffner, IEEE Photonics Technology Letters, 4, 1066 (1992).

2. C.D.Poole, N.S.Bergano, R.E.Wagner, H.J.Schulte, IEEE J. Lightwave Technol., 6, 1 185 (1988).

3. N. Gisin, J.P. Von der Weid, J.P. Pellaux, IEEE J. Lightwave Technol., 9, 821 (1991); Y. Namihira H. Wakabayashi, J. Opt. Commun., 12, 2 (1991)

4. CD. Poole, D.L. Favin, IEEE J. Lightwave Technol., 12, 917 (1994)

5. F.Ravet, B.Heens, X.Wang, J.C.Froidure, M.Blondel, Conference Digest, OFIVIC'97, National Physical Laboratory, Teddington, UK, October 1997, pp. 86-89.

48
POLARISATION-INDUCED PULSE SPREADING IN

BIREFRINGENT OPTICAL FIBRES WITH ZERO

DIFFERENTIAL GROUP DELAY

B. Huttner, B. Gisin and N. Gisin

Group of Applied Physics

University of Geneva

20 Rue de L'Ecole de Medecine,

CH121 1, Geneva 4, Switzerland

Abstract: It was recently pointed out that the addition of polarisation dependent losses to birefringent

optical fibre systems leads to non-intuitive results. In this work, we present computer simulations of

concatenations of trunks of birefringent fibres and elements with polarisation dependent losses. We show that the concatenation may have zero differential group delay over a whole range of wavelengths, but that a pulse

propagating down the concatenation may still experience significant pulse spreading. In this example, the two

main methods used for characterising polarisation mode dispersion in optical fibre systems, i.e. Jones matrices

eigenanalysis versus interferometric method give different results. This exemplifies the need for a careful assessment of the basic concepts related to polarisation effects in the presence of polarisation dependent losses.

Introduction

The birefringence properties of optical fibre systems are becoming increasingly important in telecommunication

as a limiting factor of the bit rate. The concept of Principal States of Polarisation (PSP) [1] is central to the study

of these birefringence properties. For a given optical fibre trunk, the PSP's are defined as the polarisation states for which the output polarisation is independent of the frequency to first order. For a fibre with no Polarisation Dependent Losses (PDL) the two PSP's are orthogonal. A light pulse polarised along a PSP propagates along the

fibre without distortion or broadening (if we neglect the other factors such as chromatic dispersion or non-linear

effects). Moreover, the two PSP's represent the slowest and fastest propagating pulses. Any other pulse will broaden during the propagation. The maximum broadening is given by the Differential Group Delay (DGD),

which is the differential delay between the two PSP's [2]. However, for a real trunk of fibre, the PSP's are very

unstable. Therefore, the parameter relevant to characterise the quality of the fibre trunk is the mean DGD or the RMS DGD. It was shown that this time-ensemble is equivalent to a frequency ensemble of the DGD [3]. This

frequency average is known as the Polarisation Mode Dispersion (PMD) of the fibre [4]. It is the basic parameter required to estimate birefringence-induced distortions in optical fibre systems. An alternative procedure for measuring the PMD is the interferometric method [5]. This method, based on low coherence

interferometry, gives directly the pulse spreading in a trunk of fibre, from which one can infer the PMD. This

method is not related to the PSP concept. It is now well known that these two methods give equivalent results

[6]. The basic concepts of PSP's and DGD still hold for the case of optical fibre links with both PMD and PDL.

However, we recently showed that the interaction of PMD and PDL produces rather surprising results [7]. For example, we showed that the DGD of a concatenation of birefringent fibres and elements with PDL may

produce a global DGD. which is larger than the sum of the DGD's of all the trunks. Moreover, in this case, the two PSP's are not necessarily orthogonal. Neither do they characterise the fastest and slowest pulse. Another

important conclusion of this previous work is that, like the PMD. the PDL is a statistical quantity, varying with

the wavelength [8]. For real fibre trunks, where the polarisation couplings vary with time. PDL also becomes a time dependent quantity. The relevant parameter is therefore its average, either in time, or in frequency.

In this work, we show another surprising result. We present a simulation of a concatenation of highly

birefringent fibres (denoted HiBi fibres) and elements with PDL (denoted PDL elements), which is such that the

total DGD of the concatenation is actually zero over a whole range of frequency. This seems to imply that pulses should propagate down the concatenation without distortion [4] (if we neglect all other causes such as chromatic dispersion). However, if we simulate the propagation of pulses, we see that they may still experience

49
significant broadening. This is confirmed by the interferometric method, which predicts non-zero PMD, in agreement with the broadening. This result shows that the properties of the DGD alone are not enough to fully characterise the birefringence properties of an optical fibre system, at least when PDL is present.

Formalism

We recently generalised the formalism of PSP's to include cases with PDL [7]. Here we shall briefiy recall some

results. The two PSP's are defined as the two output states, which do not depend on the frequency, to first order.

In the Jones vector formalism, this gives:

5.^,„„(«) = 5.J-r-'H>,„„((o) = -/^^„„,(a)) ; (1)

where d^^ denotes the partial derivative with respect to CO, 7" is the transmission matrix of the fibre link, and

is a complex number. Therefore, the PSP's are the two eigenvectors of d^T • 7" '

. The two eigenvalues are: = ±(5t + /ri), 5t being the group delay and r\ being the derivative (in frequency) of the attenuation of

the PSP. This shows that, when there is no PDL, the two eigenvalues are real (no attenuation), their difference being equal to the DGD. In this case, a zero DGD really means no birefringence. However, when the fibre link has PDL, the eigenvalues becomes complex. In this case, the DGD will be zero whenever the eigenvalues are purely imaginary. As we shall see, polarisation effects may still create pulse spreading.

A simple example

As a first test, we analyse a concatenation of three elements, composed of one element with only PDL (element

2) sandwiched between two HiBi fibres (elements 1 and 3). We make the usual assumptions that the attenuation of the PDL element and the birefringence of the HiBi fibres are both independent of the wavelength. Therefore,

phase and group birefringences are equal. The domain of validity of these assumptions shall set the wavelength

range in which the DGD remains zero. We obtain the analytic expression for the square of the eigenvalues:

X" = + P3 + 2p,P3 [cosha(e| ^3 ) + (1 - cosha)(e, • ){e-, e^) - /sinha • (e, ^3 )]. (2)

where is the axis of birefringence or PDL of element number / on the Poincare sphere, P| , P3 (in ps) is the

modal birefringence of elements I and 3, and a gives the PDL of element 2: = ^"^^y^ , t^^^^ and t^^^ / mm

being the transmission coefficients for the amplitude. We thus see that whenever ^1,^7,^3 are in the same

plane X" i"^^'' ^^'^^ ''^ positive (usual case), or negative, which corresponds to zero DGD. For

example, if • = -1 , e, • • = 0 , and P, = P3 = P , we get:

X' =-2p-(cosha-l)<0. (3)

This corresponds to zero DGD over the full wavelength range where our approximations (i.e. wavelengthindependent birefringence and PDL) are valid. It is easy to see that the precise directions of the axes e, , and

e. are not important. Negative X" can be obtained for a variety of setups. However, even though the DGD is

zero, the device will still cause pulse spreading. Consider a short pulse (short with respect to P, and P3),

propagating down the fibre. The first HiBi fibre separates the pulse into two components along and — e,

When no PDL is present, the second HiBi fibre recombines the two pulses exactly. This device therefore has

zero global birefringence. The effect of a PDL element can be understood as follows: a pulse polarised along

direction, say , is decomposed along the two axes 62 (axis with attenuation) and — (axis with no

attenuation). The effect of the differential attenuation is to reduce the amplitude along , thus creating a

rotation of the input polarisation. Since the two components along e^ and — experience a different rotation,

the second HiBi fibre cannot recombine the pulses fully. For the case under consideration, the output state is

therefore composed of 3 pulses separated by P . This time-domain analysis clearly explains the pulse spreading,

and the fact that it is predicted by the interferometric method.

50
0.25

0.20 PMD = 3.44 ps

c

(U 0.10

0.05

0.00

-15

i A A -10 l1i

-5 JiLi 0 Li 1

5

A * A 1 , 10 15 Delay [ps]

Fig.l : Computer simulation of the interferogram.

The concatenation is made with 27 elements: 14 HiBi fibres with 1 ps DGD, and 13 PDL elements with PDL

varying from 1 to 7 dB. The LED used has FWHM of 50 nm. The DGD is zero over the full frequency range of

the LED (see text for details).

Computer simulations

The most interesting point is that a zero DGD can still be obtained with a much more complicated setup. In this case, we cannot continue with analytic expressions, but resort to computer simulations. We analyse a concatenation of several HiBi and PDL elements, such that the global DGD is still zero. In order to achieve this, the main requirement is that the concatenation is symmetric with respect to the central PDL element, the birefringence axis of two opposite HiBi fibres being crossed (i.e. fast to slow and vice versa). Each PDL element

is sandwiched between HiBi fibres. The result of a simulation with a concatenation of 27 fibres is presented in Fig. 1 and Fig. 2. Each of the 14 HiBi fibre has 1 ps DGD, and the 13 PDL elements have PDL varying from I to 7 dB. These are rather high values, which were chosen to obtain a very clear effect. In Fig.l we present the interferogram, with a 50 nm FWHM LED. We see the series of peaks separated by 1 ps each, corresponding to the 1 ps delay of each HiBi fibre. The PMD calculated from the interferogram is 3.44 ps. In Fig. 2, we show the

actual pulse spreading, for an input pulse with 7 ps FWHM. The output pulse has a FWHM of 7.8 ps. This is in

excellent agreement with the value calculated from the interferometric PMD: a^^, "V'^in' +{PMD)'

This shows that the non-zero PMD predicted by the interferometric method is indeed translated into real pulse

spreading.

We have shown that the two main methods for measuring PMD in birefringent fibres may give different results when applied to elements with both PMD and PDL. A concatenation may have zero DGD but may still create pulse spreading, in agreement with the interferometric method. Since this concatenation is purpose-built to generate this effect, we cannot say at present whether or not this result will have consequences for real fibres. However, the mere fact that one particular setup creates this discrepancy between the two measurement methods

calls for a careful assessment of PMD characterisation when PDL is present. We are now working on making a

real concatenation of fibres with zero DGD and pulse spreading, and measuring its PMD by the two methods.

Conclusion

51
-15 -10 -5 0 5 10 15 Time [ps]

Fig.2: Computer simulation of pulse spreading

The input pulse has 7 ps FWHM, and is represented by the dotted curve. The full curve is the output pulse,

which has been broadened by the concatenation. Its FWHM is 7.8 ps, in excellent agreement with the value

calculated from the PMD measured with the interferometric method (see text). Again the DGD is zero over the whole frequency range of the pulse.

Acknowledgements

This work was supported by the Swiss Office Federal de I'Education et de la Science within the COST 241

Europeen project.

References

[1] CD. Poole and R.E. Wagner. Electron. Letts. 22, 1029 (1986).

[2] CD. Poole and CR. Giles, Opt. Letts. 13, 155 (1988).

[3] N. Gisin, Opt. Comm. 86, 371 (1991).

[4] B.L. Heffher, IEEE Photonics Techno. Letts. 4, 1066 (1992).

[5] N. Gisin, J. P. von der Weid and J. P. Pellaux, J. of Lightwave Techno. 9, 821 (1991).

[6] N. Gisin and J. P. Pellaux, Opt. Comm. 89, 3 16 (1992); B. Pemy et al.. Electron. Letts. 32, 680 (1996).

[7] B. Huttner and N. Gisin, Opt. Letts. 22, 504 (1997); N. Gisin and B. Hultner, Opt. Comm. 142, 1 19 (1997).

[8] A. Elamari et al., J. of Lightwave Techno. 16, 332 (1998).

52
Temperature Dependence of PMD Measurements

S. Corbeil and M. O'Sullivan

NORTEL (Northern Telecom) - Broadband Networks

Abstract - PMD correlation temperature is defined. A measurement of correlation temperature is described

and values of correlation temperature are reported for various optical fibre communications components. A

relationship between the correlation temperature and fibre length is empirically discovered.

I. INTRODUCTION

Polarization mode dispersion (PMD) can impair high capacity optical communication system performance.

In a non-return to zero (NRZ) system, the PMD penalty is mainly due to intersymbol interference caused

by a differential group delay (DGD) between the two principal states of polarization (or PSPs) associated

with a single mode fibre link. The DGD of a fibre Has a random value which varies over wavelength (or

frequency) as well as time. The DGD distribution is usually known to be stationary at fixed temperature

with a Maxwell probability density function. The time dependence of DGD at a particular wavelength stems from environmental conditions which in turn vary with time. In particular, temperature can contribute to DGD change. To study the temperature

dependence of DGD, we develop a method of measuring change in the DGD distribution.

II. THEORY

It is important to determine how temperature affects the variation of DGD. This change can be measured

by slowly varying the temperature of a device and, at regular temperature intervals, measuring the DGD for a given wavelength. The correlation function of DGD over temperature can be evaluated as in (1): r(Ar(A„,r)- A7)(Ar(/lo,r + AT)- A?)cfr .j)

[ {At{A,,T)- ArydT

Of course, an infinite temperature range cannot practically be achieved. In addition, multiple DGD values over a given wavelength range are usually obtained at each value of AT. The fact that the DGD process is stationary enables the DGD-temperature correlations to be averaged over an entire range of wavelengths to compensate for the loss of accuracy due to the limited temperature integral range in (1). Equations (1) and

(2) are expected to have the same result: r nAT{A,T)-ATXAT{A,T + AT)'-~AT)dAdT c,(Ar)=^

jjAr(/l, T) - AtY dAdT

In practice, the integral ranges in (2) are replaced by the finite range of T and X over which DGD

measurements are taken. The correlation temperature 7^ separating independent instances of PMD is defined to be the full-width at half-maximum (FWHM) of the temperature correlation fimction Cj{AT). Figure 1 denotes an example

temperature correlation function. (Figure 1 here).

III. EXPERIMENTAL SETUP

Using the Jones-Matrix Eigenanalysis (JME) measurement technique, an HP-8509 polarization analyzer was used to obtain DGD samples of various optical fibre devices, as described in Figure 2. Dispersion

compensating modules (DCMs), an erbium doped fibre amplifier (EDFA), and several spans of single

mode fibre (SMF) were measured. (Figure 2 here).

The samples were placed in an environmental chamber. For each device, the temperature in the oven was

set to a value above the ambient temperature in the lab, and the device was allowed to soak until the temperature was approximately constant throughout. For this purpose, a control sample was used

consisting of a tightly wound spool of SMF, with a thermocouple taped to the outside circumference.

Another thermocouple was taped to the inside of the spool, wherg no air was permitted to circulate. The

control sample was placed at an equal distance from the heat source as any other device, as shown in Figure 3. The average temperature of any device was taken to be the average of the two thermocouple

readings. (Figure 3 here).

Allowing the temperature to relax towards ambient temperature in a controlled and incremental fashion,

JME measurements over constant wavelength ranges were performed, recording the inner and outer temperatures at the start and end of each measurement (each JME measurement lasts in the order of several minutes). The scans of DGD over wavelength were then correlated two-by-two, and plotted against the

53
difference in average temperature between the two scans. The plot was then averaged over finite temperature ranges, and values of FWHM were obtained for each device. The stationarity at a given

temperature was measured and ascertained for each device, with the knowledge of the expected variance on the mean given in [3]. IV. RESULTS and DISCUSSION

The correlation temperatures are tabulated in Table 1, along with the length of fibre in each device. Table 1 . Device Correlation Temperatures

Component Fibre Length Temperature

Correlation

(FWHM)

EDFA 10m 96°C

DCF 1000m 12°C

DCM 7000m 3.4°C

SMF 20,000m 2.2°C

SMF 80,000m LOT

A logarithmic plot of Tc vs. / is shown in Figure 4 and appears to support a relation between the two:

(Figure 4 here). ^ 334

Although the PMD instance changes when Ar exceeds the correlation temperature, the stationarity of the random process may be at risk. Two different behaviours were noted in the samples measured above. In the DCMs, the mean DGDs of all measurements within the temperature range (-5 - 65°C) did not significantly

differ: the DGD was stationary over the entire temperature range. On the other hand, the mean DGDs of

the SMF spans showed a deterministic temperature dependence. If the primary objective is to determine

with some accuracy the mean DGD of a particular device, all DGD measurements must be conducted in a regime where the DGD process is stationary. The generation of an independent instance of PMD can then be done through the use of temperature excursions returning to the constant measurement temperature,

rather than simply increasing or decreasing the temperature by the correlation temperature. V. CONCLUSION

The PMD correlation temperature was defined and measured for DCMs, lengths of SMF and an EDFA.

The correlation temperature is empirically proportional to the square root of the length of the optical fibre device. ACKNOWLEDGEMENTS

The work and use of equipment and facilities would not have been possible without T. Taraschuk, K.

Visvanatha, and D. Atkinson. Acknowledgement also goes to P. Galko for helpful input.

REFERENCES

[1] CD. Poole, R.W. Tkach, A.R. Chraplyvy, and D.A. Fishman, "Fading in Lightwave Systems Due

to Polarization-Mode Dispersion," IEEE Photonics Technology Letters, vol. 3-1, pp. 68-70, Jan. 1991.

[2] F. Bruyere, "Impact of First- and Second-Order PMD in Optical Digital Transmission Systems,"

Optical Fiber Technology, vol 2, pp. 269-280, 1 996.

[3] N. Gisin, B. Gisin, J. P. Von der Weid, and R. Passy, "Flow Accurately Can One Measure a

Statistical Quantity Like Polarization-Mode Dispersion?," IEEE Photonics Technology Letters, voL 8-12, pp. 1671-1673, Dec. 1996.

[4] CD. Poole, J.H. Winters, and J. A. Nagel, "Dynamical equation for polarization dispersion,"

Optics Letters, vol. 16-6, pp. 372-374, Mar. 1991.

[5] B.W. Hakki, "Polarization Mode Dispersion in a Single Mode Fiber," Journal of Lightwave

Technology, vol. 14-10, pp. 2202-2208, Oct. 1996.

[6] H. Bulow and G. Veith, "Tem.poral Dynamics of Error-Rate Degradation Induced by Polarisation Mode Dispersion Fluctuations of a Field Fiber Link," presented at 11"" International Optics and

Optical fibre Communication - 23"" European Conf on Optical Communications, lOOC-ECOC

1997, Conf Publ. No. 448, 1997, pp 1 15-18.

54
Figure 1 .Temperature Correlation Function of a Dispersion Compensating Module

0.1 - \

i ; ; i i ; ; -

0 I i i i i ' i i i i -5 -4 -3 -2 -1 0 1 2 3 4 5 Temperature Difference AT (°C)

Figure 2. Experimental Setup

55
Figure 3. Oven Chamber Layout

Heat Source

Thermo-couples

Figure 4. Correlation Temperature vs. Device Length

Uo

S

c

_o

c3 1)

i-<

1-1 oU

Optical Fibre Length / (m)

56
Measurement of Two Dimensional Axial Stress Profiles of Optical Fibres with High Spatial

Resolution

K W Raine' and A J Parker"

' K&V Optics. UK. " National Physical Laboratory. UK.

Introduction

At the 1997 OFMC conference', Raine described an instrument developed to measure birefringent retardation

profiles, and hence axial stress profiles, in optical fibres with high spatial resolution and low noise. This instrument has now been modified to measure 2D and 3D profiles without the assumption of circular symmetry. The technique

enables each full width retardation profile to be obtained in a few seconds. At present the spatial resolution has been

limited to 0.8 \xm but will be improved when a CCD Camera with more pixels has been added. In this paper we

present the axial stress profile of a polarization maintaining fibre in order to demonstrate the performance of the instrument and describe the practical and numerical challenges that had to be overcome.

Instrumentation

The instrument was described in detail at the 1997 OFM conference', and consists of a reversed strain viewer

attached to a transmission microscope. Use of the half shadow''\ or half shade, principle leads to a linear birefringent phase profile free of the square terms of intensity and many of the background birefringent phase

differences due to component imperfections'. The half shade device was originally formed by using a mica half- wave

plate on axis, covering half of the beam only, and a polarizer rotated by the required half shade angle (H) as described by Laraunt^ in 1880. The edge of the wave plate was focused into the plane of the fibre using the microscope condenser. The intensity profiles on each side were measured by an electronic system designed to sample two vertical lines from the TV image. For 3D work it is not possible to set up the same part of the image on

the two sampling lines. Even in 2D work some care has to be taken to ensure correct image registration. To

overcome this the half shade boundary was replaced by a time-modulation of the direction of the input polarization.

This was conveniently achieved by use of a Faraday modulator consisting of a coil, of about two hundred turns, surrounding a cylindrical SF57 glass core about 10 cm long. Anti-reflection coatings were applied to both ends of

the core. Multiple reflections need to be suppressed because the Faraday effect is additive and changes in the length

of core due to temperature changes, lead to unwanted modulations. The modulator was able to produce up to a few

degrees of modulation using a 50 Watt power amplifier. A one degree modulation was chosen for this work. The

angle of polarization rotation produced by the Faraday modulator was controlled by the same computer used to acquire and analyse the video image. These modulators have very low noise making it possible to measure

polarization angles as small as 0. 1 arc sec'', for precise measurement they are usually preferable to other modulators'*.

The two signals (1+) obtained with each direction of Faraday cell modulation are shown for small angles, in the equation below:

l±oc[±H + 0.5u5 + e + 0.5P]'

Here H is the half shade angle, 5 is the birefringent phase difference, 0 is the angular rotation of the quarter-wave

plate, u is ± 1 depending on the orientation of the quarter-wave plate axes. Similarly P

is the rotation of a second

half-wave plate. The difference between the two signals (AI) is proportional to the retardation profile and is free of

squared terms present in each signal. It is given approximately by:

AIocuH5 + 2He + Hp.

Calibration of the retardation profile can therefore be achieved by measured rotations, 9 or p, of the quarter-wave

plate or half-wave plate respectively. The calibration profile contains cross-products between the rotations 9 and P and background phases, which are nearly identical to those formed with the sample birefringence'; for simplicity these are not shown in equation above. The calibration therefore corrects some phase errors. Similarly a change in the sign of u reverses the sign of the signal but not signals arising from background phase imperfections and the effective birefringence of the aluminised mirror. This too, can therefore be used to remove instrumental phase

57
errors'. Alternatively, if the half-wave plate is used for calibration, a measured background phase profile must be

subtracted to remove background phase errors. The advantage in using the half-wave plate is that only one measurement of the sample is needed, which is desirable when many views of the sample are required. The

disadvantage is that the calibration uncertainty is increased. Thus the method and measurement procedures allow

phases as small as 0.005° to be detected in the presence of background phases many times greater". A more complete

description of the instrument will be published later.

As can be seen in figure 1 the Faraday effect modulator was placed after the polarizer in the weakly focused beam.

At the point of entry the beam diameter was smaller than the core diameter, so all of the light was guided by the core. The core was supported on two narrow rings, about 1 mm wide, near to the ends, to minimise losses due to frustrated

internal reflection.

Prior to sampling the polarized images an additional polarizer, not shown in figure 1, is temporarily placed between

the instrument polarizer and the Faraday modulator then aligned nearly parallel to the transmission axis of the analyser. This removes the polarization sensitivity and the resultant intensity profile can then be used to normalise the phase profiles for the intensity distribution of the illumination. It is also possible to remove the polarization

sensitivity ft'om the image by making measurements with zero modulation angle. However, this produces low signal values making the normalisation process too noisy.

The numerical aperture of the objective was 0.5 and the condenser was set to have an effective numerical aperture of

0.3. The analyser was high quality polarizing sheet that had a negligible effect on the wavefront.

It was necessary to rotate the fibre to generate different projections to form the 2D axial stress profile from the 2D

phase profile. This leads to some practical problems. As a rough guide the axis of rotation should not move by more

than about one tenth of the spatial resolution if artefacts from the transform are to be avoided^ This amounts to approximately 0.1 |im for the instrument described here. The fibre must also rotate uniformly with no stiction. Alternatively if measurements are made through 360° instead of 180°, the apparent sampling redundancy can be used

to remove the artefacts though this produces a profile with an apparent poor focus^

The coated part of the fibre was fixed into a thin steel tube that was connected to a computer-controlled rotation

stage. The bare fibre was passed through two ceramic ST fibre connector ferrules whose coned ends were

approximately 1 1 mm apart. These were located in two aligned 'V grooves. The steel tube rotated in a plain bearing.

The free part of the fibre passed over a small section of microscope slide. A glass cover slip was positioned above

the fibre but not touching it. The fibre was surrounded in a silica index matching fluid. Movement of the fibre axis can be observed directly if there is stress at the outer edge of the cladding. Sticking and twisting of the fibre can also be monitored by watching the movement of small particles of dirt on the fibre surface.

Results

As a trial run, a bow-tie polarization maintaining fibre was measured by taking 144 projections around the full 360°

rotation, each having a spatial sampling interval of 0.5 |am. Each projection was made by averaging 30 camera

frames and over 12 pixels (6 fim) along the axis of the fibre, i.e. perpendicular to the sampling direction; each

projection thus took a few seconds to measure.

The retardation data was analysed in two 180° sections using a least squares iterative technique^. The individual retardation profiles were numerically centred using the edges of the fibre, assuming that the fibre was elliptical. The

second 180° data set was used to average out any remaining errors^ The optimum number of iterations was gauged

by comparing the retardation profile, calculated from the transformed stress profile, with the measured retardation

profile. Allowing the iterative process to go on too long can lead to the enhancing of noise due to rounding errors. The results are shown in figures 2, 3 and 4. The axial stress profiles show less than 2 MPa noise for the cladding,

increasing to 5 MPa at the core centre. The core of the fibre is resolved. A value of 3.5x10"'' MPa"' was used for the photo-elastic constant.

In this case the fibre centre only moved by about 1.5 |im, but movements of 10 ^im have been observed during other measurements. Cleanliness of the fibre and ferrules as well as the correct selection of fibre ferrule internal diameter

58
are important to enable smooth rotation of the fibre and prevent fibre twist. As can be seen in figure 5 the effect of using uncentred data is to increase the noise, especially in the centre of the profile and introduce artefacts in the calculated stress profile. The exaggerated extension of the compression at the edge of the bow tie is probably due to decentring.

Care must be taken in quoting a spatial resolution as it will depend on the numerical aperture of the objective and

condenser in a complicated way. The off-axis rays within the numerical aperture of the objective are effectively

making different projections, with different retardations, which will be averaged out in the image. For example, this will have the effect of smoothing out sharp features. We intend to model this effect so that the effective spatial resolution can be estimated for given numerical apertures assuming optimum sampling intervals.

The use of alternative techniques for analysing the data, specifically the Back Projection Fourier Transform method*

will also be investigated.

Conclusion

An instrument for measuring the 2 dimensional stress profiles of optical fibres has been presented. It offers increased

spatial resolution, reduced noise and efficient data collection compared with other published methods. At this early

stage we cannot estimate the overall uncertainty of the measurements and transformations as sources of systematic

errors have yet to be fiilly investigated.

1 K W Raine. 'A microscope for measuring axial stress profiles with high spatial resolution and low noise'. Proc. 4'^ Optical Fibre Measurement Conference (NPL Teddington UK), 269, (1997)

2 R S Longhurst, 'Geometrical and Physical Optics', Longmans, 1964, p485

3 C J Koerster, "Optimum half shade angle in polarizing instruments.", J. Opt. Soc. Am, 49 (6), 556 (1959)

4 R J King and K W Raine, 'Polarimetry applied to alignment and angle measurement' Optical Engineering,

20, p39, 1980

5 The Image Processing Handbook Chap. 12, Second Ed. John C. Russ. CRC Press 1995.

6 Three-dimensional Reconstruction in Nuclear Medicine Emission Imaging. T.F. Budinger and G.T.

Gullberg, IEEE Transactions on Nuclear Science Vol. NS-21, June 1974. pp2-20.

I I CCOcom^o

59
Figure 3. As figure 2 but viewed from other

side showing compressive axial stresses. Maximum values about -100 MPa.

Figure 4. Two orthogonal axial stress profiles taken from figure 2.

Figure 5. Axial stress profile calculated from uncentred data taken from projections over 180 degrees.

60
Biased perturbation method for index profiling single-mode fibers

fi:om near-field and far-field data

N. H. Fontaine and M. Young

Optoelectronics Division 815.02, National Institute of Standards and Technology,

325 Broadway, Boulder, CO 80303

We have developed a biased perturbation method for calculating the index profiles of singlemode fibers from either near-field or far-field data. Unlike other perturbation and direct inversion

methods, which require smoothing of the near-field data, our method works by smoothing the

perturbation. This biased perturbation method minimizes the loss of resolution and distortion

at large radii and forces the index profile to converge to the cladding index. We have compared

index profiles with those measured by the refracted near-field method, showing agreement within

An ~ 0.0005 from the fiber axis to a radius of 20 jiva.

THEORY

Under weakly guiding conditions the mode-field of a single-mode waveguide is closely

described by the scalar wave equation [1]. The square of the refractive index profile

n] = [n {fi)^ of the guide may be determined from a discrete set of near-field data = u{fi)

by direct inversion of the discrete wave equation [2]

V?^.. + (A:^n? = 0 = - ^V]u,. (1)

The mode-field Ui is the square root of the measured intensity dzyl{ri)^ (3 is the propagation

constant, and V^^ is the discrete form of the Laplacian at the point r\. The discrete

Laplacian V^Wj is very sensitive to noise and spatial measurement errors of the field w^,

which are amplified in the term V^Wj/uj wherever nears 0. The amplification of errors

results in large fluctuations of the index profile [2-4]. Frequently, the near-field data are smoothed to reduce the effects of errors on the Vfui term [3-5]. However, smoothing the nearfield data can diminish the spatial resolution and distort and bias the index profile. The

direct inversion method also requires a separate determination of the propagation constant

(3. While /3 shifts nj [4,6], it both shifts and scales the index rii as [3]

In principle, j3 may be determined by making the index profile equal to that of the cladding

index at large radii. However the index fluctuations make this determination very difficult.

The perturbation mode-field [4,6] is formed by writing trial function £o,i plus a sum of increasing mth-order perturbed fields Em,i- = T^^'o,? + T^-^'i.i + 7^^2,z + • • • The

material parameter and propagation constant P are expanded as nf — 7°?^o,2 + T^'^i,? +

7^^2,i + • • • — l^Pl + 7^/^i + l^Pl + • • -5 repectively. Substitution of the expansions

into the wave equation of Eq. (1) and separating powers of 7 gives to first order

The trial function Eq^i is determined by solving a simplified wave equation using a priori

knowledge of the approximate form of the trial index profile no,^. The constant (Sq is determined by analytical methods, such as solving a characteristic equation for the mode-field.

61
The first-order field is the difference Ei^t — u-i, — Eq^i between the data and the trial

function, which makes the perturbation term

7^ : nl, = ^ + (u,V]Eo, - Eo,V]u) . (4)

Choosing the measured data as the trial mode-field (i?o,2 = w^) reduces the perturbation

method to the direct inversion method. Therefore, if it were possible to obtain noise and

error free data and to determine P, a perturbation method with Eqj ^ could never lead

to an index profile with greater accuracy than that obtained by direct inversion.

APPLICATION TO SINGLE-MODE FIBERS

We sought to compare the 1-dimensional direct inversion and perturbation methods to

the index profile obtained by the refracted near-field method on a single-mode fiber [7]. We

measured the far-field diffraction pattern of a single mode-fiber at a wavelength of 1.310 yum

and Hankel-transformed these data to the near-field Ui [8]. We measured the index profile of a similar fiber by the refracted near-field method at 0.635 fxm. The concentration-dependent

dispersion was determined by Sellmeier's equation to be 0.00025 greater at 0.635 /im than

at 1.310 /im when the dopant concentrations changed from 0 to 13.5 mole% Ge02 in Si02

[9]. We ignored this small correction to the index and simply shifted the entire index profile

measured with the refracted near-field method at 0.635 /im to match the Si02 cladding index

of rzB = 1-4468 at 1.310 /xm.

We constructed a simple trial field Eq^^ by matching the step-index solutions AJo{KAr)

for r < a and BKo{KBr) for r > a and their slopes at the core/cladding boundary r = a. The

constants k are ka,b = \/|^^^a,b "~

/^o I- The trial function was determined by: choosing trial

parameters {nA,r = a} and finding the effective propagation constant (Sq that minimized

the characteristic equation ^ _ Ko{KBr)dJo{KAr)/dr - Jo{KAr)dKo{KBr)/dr

^ _ ^. Ko{KBr)dIo{KAr)/dr - Io{KAr)dKo{KBr)/dr

using the value of (3o to match the solution at the boundary ^ _ ^

Io{KBr)dJo{KAr)/dr - Jo{KAr)dIo{KBr)/dr ^ _ ^. Ko{KBr)dIo{KAr) / dr - Io{KAr)dKo{KBr)/dr

repeating the process to diminish the error a = J2t l^iA"^ much as possible (Fig. 1(a)).

The zeroth-order perturbation, first-order perturbation, and direct inversion solutions for

the index profile are shown in Figure 1(b). The Hankel transformation of the discrete farfield data set introduced oscillations in and these were transferred to the index profile.

The error was amplified as \ui\ (and |£^o,i|) decreased.

BIASED PERTURBATION METHOD

Since the trial function Eo^i of the perturbation method was piecewise analytic and

independent of fluctuations in the data, Hq was piecewise analytic and independent of

fluctuations as well. Therefore, the perturbation term contained all of the errors due

to the measurement and the far-field transformation. We smoothed the perturbation term

n^ - and forced it to converge to 0 for large Tj by adding a small positive bias S,, to the

denominator in Eq. (4): Eq^ —> Eq- S-j,. The bias term was 8i = 0 for Tj < 4.5 /im and

62
5i = 0.01 for r, > 4.5 ^m. We compared the index profile computed from the biased direct

inversion and biased perturbation solutions

biased direct inversion:

biased perturbation: Uq

"IThe biased direct inversion method greatly distorted the index profile in the cladding and did

not converge to the cladding index (Fig. 2(a)). The biased perturbation method, however,

smoothed the Hankel-transformation error and forced the index to converge to that of the

cladding (Fig. 2(b)). The biased perturbation result agrees with the refracted near-field

measurement within Aui < 0.0005 for 0 < r,- < 20 fim. The remaining fluctuations can be

reduced in the biased perturbation method by increasing Si at the expense of distorting the

perturbation contribution to the index profile.

CONCLUSIONS

The perturbation method can be superior to the direct inversion method when smoothing

is applied to the perturbation term . The biased perturbation method is superior to both

the direct inversion and unbiased perturbation methods because the index profile can be

forced to converge to the cladding index. Far-field diffraction data may be used to obtain

very accurate index profiles of single-mode fibers. In addition to the work presented here, we

have also shown through simulations that the biased perturbation method will substantially

reduce the effects of noise in the mode-field measurement on the calculated index profiles.

We plan to extend these analytical methods to two dimensions to enable calculation of the

transverse index profiles of rectangular waveguides from two-dimensional far-field data.

REFERENCES

[1] C. Yeh, L. Casperson, and W. P. Brown, "Scalar-wave approach for single-mode inhomogeneous

fiber problems," Appl. Phys. Lett., 34(7) 460-462, 1979.

[2] Leon McCaughan and Ernest E. Bergmann, "Index Distribution of Optical Waveguides from

Their Mode Profile," J. Lightwave Technol., LT-l(l) 241-244, 1983.

[3] Jochen Helms, Joachim Schmidtchen, Bernd Schiippert, and Klaus Petermann, "Error Analysis

for Refractive-Index Profile Determination from Near-Field Measurements," J. Lightwave Technol.,

8(5) 625-633, 1990.

[4] Katsumi Morishita, "Refractive-Index-Profile Determination of Single-mode Optical Fibers by a Propagation Mode Near-field Scanning Technique," J. Lightwave Technol., LT-1(3) 445-449, 1983.

[5] I. Mansour and F. Caccavale, "An Improved Procedure to Calculate the Refractive Index Profile

from the Measured Near-Field Intensity," .1. Lightwave Technol., 14(3) 423-428, 1996.

[6] G. L. Yip, P. C. Noutsios, and L. Chen, "Improved propagation-mode near-field method for

refractive-index profiling of optical waveguides," Appl. Opt., 35(1) 2060-2068, 1996.

[7] M. Young, "Optical fiber index profiles by the refracted-ray method," Appl. Opt., 20(19) 3415-

3422, 1981.

[8] M. Young, "Mode-Field Diameter of Single-Mode Optical Fiber by Far-Field Scanning,"

Appl. Opt., (in press).

1

fc2 k^U^ + 5

Pi 1 k' k^Eo,

(7)

1 P k^'El^ + 6

(u.V'iEo^^ - £o,.Vfw,) , with /?i = 0.

63
[9] J. W. Fleming, "Material and Mode Dispersion in Ge02-B203-Si02 Glasses," J. Amer. Ce- ram. Soc, 59(11-12) 503-507, 1976.

Figure 1(a). Far-field data Hankel-transformed to the near-field Ui (circles) and the

trial function £^o,2 determined by iterative fitting on the parameters {nA,a} (solid line).

We used the fused silica index ub — 1.4468 at 1.310 jim. We determined the effective

propagation constant [Sq = 6.953 /um~^, the effective core index = 1.4520, and the

effective core-cladding boundary a = 4.177 fim when fitting the trial function to the modefield, (b) Zeroth-order step-index profile no,i (straight lines) calculated from the trial mode

field Eo^i. The perturbation method (circles) gives a similar index profile to that of the

direct inversion method (solid curve). The direct inversion method does not determine

leading to an unknown shift of l3/k and scaling of l/{2f3k). Here we have chosen a value of

l3 = 6.954 ijm-\

I

I >I II

I

I

I I I

I

I I I

I

I I 0

r(nm)

10 20

Figure 2 (a). Biased direct inversion method (connected circles) and the refracted nearfield data (solid curve). Biasing fails to force convergence and distorts the index profile in

the cladding, (b) Biased perturbation method (circles) and the refracted near-field data

(solid curve). The index profile is smoothed and converges to that of the cladding index.

64
High precision fiber group refractive index measurement using an

interferometric method

Gisella Marradi, Pier Giuseppe Peretta, Susanna Cattelan

SIRTI S.p.A. Cables and Optical Technologies

Via E. Fermi, 2-20060 Cassina de' Pecchi (M)- ITALY

Abstract: Using high spatial resolution and large dynamic range Optical Low Coherence

Reflectometry (OLCR), group refractive index can be determined by precisely measuring

light optical path difference between a test 'fibre sample and a reference one.

1. Introduction

Using high spatial resolution and large dynamic range Optical Low Coherence Reflectometry

(OLCR), group refractive index ng can be determined for the fibers which ng unknown is. Group

refractive index is an interesting parameter who allows us to measure the light optical path in a

fiber and therefore its geometrical lenght.

In the following pages our relative measurement procedure is described: group refractive index ng

is obtained comparing the light optical path between a test fiber sample and a reference one.

2. Theory

In this section the theory on which the technique we used to measure ng is based and the error analysis are described.

Our aim was to measure the group refractive index ng of an 1, lenght fiber using a knwon

refractive index nj of I2 lenght standard SMR fiber as a reference.

Using an OLCR device is possible to measure the light optical path x in a fiber.

Refering to the reference fiber we obtain :

Refering to a test fiber:

x,=hn^ (2)

Combining equations (1) and (2) we obtain:

Supposing l,=l2 and knowing of the reference fiber, ng can be easily calculated by means of the

experimental measurement of the two optical paths.

At this point we have to define how much the length difference between the two fibers affects the

ng value.

Let's modify equation (3) as follows:

/| X, X|

The errors related the parameters in equation (5) are the followings:

• I = ~ = l±Sl where SI is the cutting error;

• X, + e + (Sc, ; X, and can be considered equal as they depend on the OLCR precision

which is >5|im;

n^ = '-^-n,-^ = l-n,-^ (4)

65
• ±Sn^ = 1.468 ±2 -10" , according to the typical values for the third window supplied by

SMR fibre manufacturers (see Table 1). Computing the known error propagation equation and assuming 1^=1-,= \m we would obtain:

Case a) 51 = 10'^ that is \mm, 5n^ = 2.8 • 10"^

Case b) ^/ = 10~* that is 1,-1,= OAmm, 5n^ =2-10"'

We conclude that by assuring a cutting error smaller than O.lmm/m, we reach the reference fiber

precision ( « = 2 • 10"\ see Table 1).

Fiber type 1300 nm ng 1550 nm

SM-R 1.467 1.468

SM-DS 1.471 1.470

Table 1- Nominal values (error of ±2 on third figure) for the refractive index of SMR and DS fibers

3. Experimental setup

All the group refractive index measurements have been performed with a OLCR implemented in

the Sirti R&D labs [1,2]. Figure 1 shows the automated experimental test-bed, which uses low

coherence sources (LEDs, ASE) to obtain a high resolution Michelson interferometer. The 3 dB

coupler splits the light launched into the SMR fiber into two branches: the measurement and the

reference one. Light is reflected by a mobile mirror placed at the end of the reference branch and

in the measurement branch by the fiber under test connected at its end. We can calculate the

optical path in the fiber under test by means of the interference peaks generated from both the

fiber ends and the mirror at the detector interface.The dashed line in figure 1 shows that

reference branch length can be varied, through special patch cords, in order to match different

fiber lengths.

Mesurement

branch\

Reflection at the

fiber- fiber interface Reflection at the

fiber-air interface

Test fiber J

Figurel: experimental setup of OLCR developed by SIRTI

66
4. Measurements

4. 1 Cutting error evaluation

By using two SMR fibers with known ng one can easily find out the lenght difference from the

distance between the interference peaks due to reflections generated at the fiber ends, hi the ideal case of a perfect cut, the peaks overlap.

We have developed (Sirti R&D laboratories) a "double cleaver" device; it is based on two

standard fiber cleavers and enables to cut the two fibers at the same time. OLCR measurements on

several cutting trials showed that our double cleaver always keeps the length difference between

the two fibers inferior to 0. 1 mm/m . In this way the theorical precision expected on ng (2x10"^) can be reached.

4.2 ng measurements

After having verified the feasibility of the experimental method (& 4. 1 ), we carried out a number

of ng measurements on different fibers: Single Mode fiber (SM), Single Mode- Dispersion Shifted

fiber (DS), Multi Mode fiber (MM), Doped fiber, DISCO fiber.

The measurement procedure is the same as in § 4. 1 , except that the two fibers are different. The

first is a SMR fiber with a known ng value; the second is the fiber under test with a ng value which

must be measured. OLCR measures the optical path within each fiber: comparing these two values

we can calculate the test fiber group refractive index (equation 4).

The mean values of the results are reported in Table 2. They are in agreement with the ones

obtained in [3,4] using a numerical method.

Disco 1 Disco 2 SM-DS 1 SM-DS 2 Multi Mode Doped SM

1.480 1.479 1.471 1.471 1.478 1.477 1.468

Table 2: mean values of for seven fibers

As an example we report in figures 2, 3 some graphics showing the two interference peaks related

to the fiber-air interface of the reference SMR fiber and the test DS (SM,..) one. In fact the fiberfiber interface interference peaks are not shown because they overlap.

0

J.

-20 .-

-140 J , 1 1 1 1 ^ 1 ,

1 191.75 191.8 191.85 191.9 191.95 192 192.05 192.1 192.15 192.2 Position (cm)

Figure 2: position difference between the reflection at the test DS fiber-air interface and the reflection at the reference SMR fiber-air interface

67
0

-20 .-

-140 -I i 1 , , , 1 192 192.05 192.1 192.15 192.2 192.25 192.3 Position (cm)

Figure 3: position difference between the reflection at tlie test SM

fiber-air interface and the reflection at the reference SMR fiber-air interface

5. Conclusions

This article demonstrates that we can use the Optical Low Coherence Reflectometry technique to measure the fiber group refractive index ng with an error of 2xl0'\ is evaluated by measuring

the optical path difference between the test and the reference fiber having a lenght difference

inferior to 0. 1 mm/m.

The ng experimental measurements satisfy the theoretical previsions and are in agreement with

literature results obtained through a different method.

The ng precision cannot be better than 2x1 0^ that is the known ng precision of the reference SMR

fiber.

REFERENCES

[1] BottanelH & all "A new wide scan range high resolution Optical Low Coherence Reflectometer",

Symposium on Optical Fiber Measurements- Boulder (USA)

[2] Bottanelli & all "Wide range Optical Low Coherence Reflectometer for high resolution fault detection and

component characterization", 45'*' hitemational Wire and Cable Symposium -IWCS 96

(USA)

[3] W.V. Sorin, D. F. Gray

"Simultaneous thickness and group index measurement using Optical Low-Coherence

Reflectometry", lEE Photonics technology letters. Vol. 4, No 1, January 1992

[4] S. Morasca & all "Measurement of group effective index in integrated semiconductor optical waveguides",

lEE Photonics technology letters, Vol. 5, No 1, pag. 40-42, 1993

68
Are the Formulas for Mode-Field Diameter Correct?

Ronald C. Wittmann, Electromagnetic Fields Division

Matt Young, Optoelectronics Division

National Institute of Standards and Technology

325 Broadway, Boulder, Colorado 80303

We have found conceptual problems with the far-field formula^ for mode-field diameter.

First, the formulation is incomplete in that it ignores the obliquity factor; second, it assumes that

the field at the end of a truncated fiber is identical to the mode field of the fiber. When both these

factors are taken into account, the resulting integrals fail to converge. We suggest that the

original far-field formula be taken as the definition of mode-field diameter and derive an alternate

near-field formula.

It is important not only to avoid the paraxial approximation, but especially to avoid using a hybrid of paraxial and nonparaxial. Thus, we write for the far-field diffraction integral,

1*00 U{p) = 2nO{e)\^u^{r)^o{2Kprydr, (1)

where uj^r) is the amplitude of the scalar field on the plane z = 0 (the aperture plane), U(p) is the

scalar far-field pattern, and p = sind / A. See Figure 1 . The notation is that of Gaskill' We

assume here that u^(r) is independent of azimuthal angle and that scalar theory is adequate; these

assumptions are only approximately correct,^ and both depend on the assumption of weak

guidance. We discuss the factor 0(6) below. Note that p = smd / A, not 6/ A.

Let us write ( 1 ) in the shorthand form

Uip) = O{0)m[u^(r)l (2)

where HT means Hankel transform. The inverse Hankel transform is

u^{r) = 27r\^ [U{p) I OiO)]] Q{27n-p)pd p, (3)

or u^{r)^m-\U{p)IO{e)l (4)

so the Hankel transform pair consists of iij^r) and U{d)IO{d), not uj^r) and U{d). In what

follows, we will ignore evanescent waves; that is, we will ignore values of p > \/A on the grounds

that they are insignificant as - °°. This limits the resolution in the aperture plane to approximately one wavelength.

The factor 0{d) is called the obliquityfactor or the inclination factor, and it is equal to 1 in the paraxial approximation. It is often overlooked, even when the calculation is thought to be

nonparaxial. This is so in part because the optics literature has sometimes been confijsed as to the

fiinctional form of 0{d). The correct form, however, was derived by Sommerfeld"* and is well

known to electromagnetic theorists, though their notation and terminology are different.^

Specifically, when the aperture field is known, the obliquity factor is^

69
O{O) = cos0, (5)

not' 0(^) = (l + cos6')/2. (6)

Petermann has derived a useful expression for the mode-field radius of a fiber:**

u'o^P^ =[2jJ|z/n,(r)prcyr/Jja/m(r)/^i-rdr]^/\ (7)

where nj^r) is the mode field of the fiber and differs fi'om the aperture field u^{r). Artiglia and

coworkers used (2) and (7) with 0{6) ^ 1 to derive the relation^

WQ ^ (A / 27r)[2r I(O)sm0cos0d0/r^^ l{0)sm^ Ocosddof^, (8)

JO JO

where I{6) = U^{6) is the intensity measured in the far field of a truncated single-mode fiber. Equation (8) is used in the TIA standard for measuring mode-field diameter by far-field

scanning. '° Equation (8) is not quite correct: first, because it sets the obliquity factor equal to 1 and,

second, because it assumes that the aperture field is the same as the mode field of the fiber.

We can easily correct the first defect by applying (5) and replacing I{d) with I{d) Ico^d:

vvQ^P') = (A/ 2;z-)[2jJ'

"^^7(^)100 6'd^/j^''^^/((9)sm^^tan(9d6']^''^. (9)

This is (7) transformed to the far field, except that the aperture field uj^^r) has to be used in place

of uj^r); hence, the superscript (P) is replaced by (P')- Equation (9) follows rigorously from

applying (3) to the aperture field and could therefore be considered the "correct" result for scalar

theory.

The integrals in (9) diverge unless the limit of integration is less than ti/2 (see below). The

calculated resuh therefore depends on the upper limit of integration, and it may be difficult to decide where to set that limit if the fiber has a particularly broad far-field pattern. Nevertheless,

to assess using (9) in place of (8), we calculated the mode-field diameters of a step-index fiber and

a dispersion-compensating fiber using both equations, that is, both with and without the obliquity

factor^' Including the obliquity factor decreases the calculated mode-field diameter, because

dividing I{d) by cos"^^ effectively broadens the far-field pattern and narrows the near-field pattern

accordingly. With the step-index fiber, the decrease was 20-25 nm and did not change much with

angle of scan, because the far-field intensity is small when angles are large. See Figure 2. If we

could extend the integration to 7r/2, however, the integrals would diverge.

With the dispersion-compensating fiber, the decrease grew from 80 tolOO nm as the angle

of scan increased fi'om 26 to 40°. That is, including the obliquity factor in the calculation may

change the result by perhaps -25 nm with a common step-index fiber but possibly by -100 nm or more with a particularly small-cored fiber

The difference between the two calculations, including or not including the obliquity

factor, is not a component of uncertainty. To the contrary, if the calculation that includes the

70
obliquity factor is correct - or more nearly correct than the simpler theory - then the simpler

calculation gives rise to a systematic error.

The discontinuity at the end of the fiber excites waves that are radiative in the hemisphere

z > 0 and whose amplitudes fall off as \/R in the hmit °°. In the aperture plane, R = r\ if we

apply (7) to the aperture field with uj^r) in place of uj<^r), we find, for example, that

o ^ 1 j|wa(r)|-rdr-^|-yf'dr->ln(r), , (10) r

which diverges as r - The denominator similarly diverges, so (7), when applied to the aperture

field, is indeterminate.

The integrals in (9) likewise do not converge, because of the tangent terms. The integrals

in (8), by contrast, converge but do not correctly follow from (7) when (7) is applied to the

aperture field. We therefore suggest that (8), though arguably incorrect, be adopted as the

definition of mode-field diameter in place of (7). This has advantage in that is consistent with

present practice and gives at least a basis for comparing different fibers.

How can we correctly transform (8) to the near field? Let the field a distance r > 0

beyond the aperture plane be u{r, z), where z = 0 is the aperture plane and iij^r) = u{/\ 0). See

Figure 1 . In terms of the field in the plane defined by z, the far-field pattern may be expressed as ^(^)exp(iA:^z) = 0(^)HT[«(r,z)], (11)

where k^ = k cos ^ is the z component of the propagation vector. Inverting ( 1 1) we find

u(r,z) = m~\UiO) I C>(^)exp(i k^z)]. (12)

Taking the derivative of both sides of (12) with respect to z brings out a factor of k, = k cos 6 on

the right side; the factor cos ^cancels the divisor 0(6). Apart fi"om a constant factor, cit{r, z)IA

and U{d) are a Hankel transform pair, whereas u{r, z) and U{d) are not. This consideration leads us to the expression

vi;o-[2|^''|z/a^)P^^'"/|Q^I^4(^)/^l^^dr]^/^ (13)

where u^{r) = [du{r, z)ldz] ^ = q is the normal derivative of the aperture field. In the region near the

core, the aperture field is very nearly a TEM wave (and is probably very nearly equal to the mode

field), so (13) reduces to Petermann's original definition (7). Equation (13) converges, however,

whereas (7) will generally not converge if the mode field uj^r) is replaced by the aperture field

itj^^r). The vector equivalents of (7) and (13) may be obtained by carrying out the replacements u - |E|, duldr - \dY.ldr\, du/dz - \dE/dzl and so on, where E is electric field.

Whether Wq is usefiil for characterizing a fiber is not clear. Applying (13) experimentally

to near-field scans may be difficult when the aperture field is not very nearly a TEM wave, since

the near-field scan measures intensity as a fianction of position, whereas we need the axial

derivative of the field to use (13). Thus, we should expect discrepancies between near- and farfield measurements of Wq.

71
We are indebted to Andy Hallam of PK Technology for pointing out the problem and to Dave Hill and John Schlager of NIST for their carefijl reading of the manuscript. Not subject to copyright.

-40 -20 0 20 40

Angle, "

Figure 1 . The vicinity of the aperture plane Figure 2. Far-field intensity of a dispersion-

(the exit face of the fiber). compensating fiber and a step-index fiber.

References.

1 . M. Artiglia, G. Coppa, P. Di Vita, M. Potenza, and A. Sharma. "Mode field diameter measurements in singlemode optical fibers," IEEE J. Lightwave Teclmol. 7, 1139-1152 (1989).

2. Jack D. Gaskill, Linear Systems, Fourier Transforms, and Optics. Wiley, New York, 1978.

3. Matt Young, "Mode-field diameter of single-mode optical fiber by far-field scanning," Appl. Opt., in press.

4. Arnold Sommerfeld, Optics, Lectures on Theoretical Physics, Vol. IV, Academic. New York, 1964, Sect. 34.

5. D. M. Kerns, Plane-Wave Scattering Matrix Theory ofAntennas and Antenna-Antenna Interactions, Nat. Bur. Stands. (U. S.) Monogr. 162 (1981).

6. Matt Young and Ronald C. Wittmarm, "Vector theory of diffraction by a single-mode fiber: Applicafion to mode-field diameter measurements," Opt. Lett. 18, 1715-1717 (1993).

7. M. Bom and E. Wolf, Principles of Optics, Macmillan. New York (1964). Sect. 3.8.2.

8. K. Petennann, "Constraints for fundamental-mode spot size for broadband dispersion-compensated singlemode fibres," Electron. Lett. 19, 712-714 (1983). C. Pask, "Physical interpretation of Petermann's strange spot

size for single-mode fibers," Electron. Lett. 20, 144-145 (1984).

9. Artiglia, Ref 1.

10. Anonymous, "Measurement of mode field diameter of single-mode optical fiber." Fiberoptic Test Procedure

FOTP-XX, Telecommunications Industry AssociaUon, 2500 Wilson Boulevard, Suite 300, Arlington, Va. 22201-

3834, in preparation.

11. Young, Ref 2.

72
Simultaneous measurement of relative outer diameter along fiber axis:

nm resolution in diameter and mm coverage in length

Andrew W. Poon and Richard K. Chang

Department of Applied Physics and Center for Laser Diagnostics

Yale University

New Haven CT 06520-8284

I. Introduction

Uniformity of cladding diameter along the fiber axis is one of the critical parameters for

successful splicing of optical fibers. Conventional techniques for measuring outer diameter

involve counting the number of fringes in the far-field forward scattering pattern of a fixed

wavelength laser beam.' Other precise methods based on resonant light scattering from fiber

have also been reported.23 However, these outer diameter measuring techniques only sample

discrete positions along the fiber axis, and multiple measurements are required to characterize the

diameter along the fiber. A simultaneous measurement of relative outer diameter along the fiber

axis may therefore provide a more rapid and accurate in-line characterization of the fiber

diameter uniformity. In this Symposium, we report a new technique that allows nm resolution in

diameter and mm coverage in length, that is based on analyzing, at a fixed scattering angle, the

morphology-dependent resonances (MDR's) in the elastic-scattering spectrum as a function of

both wavelength and positions along the fiber axis.

Light that is incident perpendicularly to the fiber axis couples into the fiber and travels

around the circumference via successive total internal reflections at the cladding-air surface. The

circulating internal wave is on resonance when the internal wave after each round-trip is in-phase

with the incident wave. The resonant wavelengths are therefore highly dependent on the fiber

diameter, while the linewidth of the resonances are sensitive to the fiber surface roughness.

MDR's in the light elastic-scattering from an optical fiber have been demonstrated in previous

experimental studies.2.3 The spectrum revealed sharp discrete resonance peaks. The positions of

the resonances are insensitive to the scattering angle, but the intensity of the resonances is

scattering angle dependent. The MDR's may ride on broad linewidth peaks associated with

Fabry-Perot resonances between fiber surfaces at a high order rainbow scattering. The two

polarization modes, TM and TE, have different sets of resonances.

73
II. Experiment and Results

We employed a new high-resolution technique to study MDR's along the fiber axis in

commercially available optical fiber (Coming Inc.) with 125 jim nominal diameter. The plastic

coating on the fiber is chemically removed by applying methylene chloride and rinsed with

spectro-grade acetone to avoid introducing scratches that result from mechanical stripping. The

fiber was illuminated perpendicularly to its axis with monochromatic light from a continuously

tunable external-cavity diode laser (New Focus model 6202) with wavelength from 664 to 683

nm and linewidth ~ 1 0'^ nm. The wavelength was linearly scanned and our data-taking routine

allowed a wavelength resolution ~ 0.01 nm. The incident and scattered polarization were set

horizontally to the fiber axis (TE mode). Light was focused weakly by a ~ 14 cm cylindrical lens

into a ~ 5 mm long sheet that was collimated along the fiber axis. Elastic-scattering at 90°

scattering angle was detected by an intensified silicon photodiode array detector (EG&G model

1420) mounted with the array axis parallel to the fiber axis. This detector orientation allows

simultaneous measurement of the resonant spectra along the fiber axis.

Figure 1 (a) shows a series of elastic-scattering spectra in wavelength along an estimated 2

mm length of fiber. The scattered light intensity profile only spanned ~ 1 00 channels of the 1 000

channels available in the ~ 2 cm array detector. Figure 1(b) shows the same set of data but

plotted with the "head-on" view in order that the resonant wavelengths at different positions

along the fiber axis can be visually compared. A series of five MDR's was observed with

linewidth ~ 0.07 nm to < 0.03 nm. The sharper but weaker MDR's appeared to be resolution

limited by our scanning technique.

Relative fiber diameter measurements are determined by relative shifts in the resonant

wavelength. For both MDR's and Fabry-Perot resonances, the relative fiber diameter AD and the

shift in resonance wavelength AA, are related by simple ratio as follows:

AD = — AA, (1)

where D is the 125 jam nominal diameter, and X is the resonant wavelength. The resolution in

relative fiber diameter measurement is limited only by the linewidth of the MDR's. For an MDR

with linewidth 0.03 nm, a shift in AX as small as 0.03 nm can be readily discerned. For the

resonant wavelength X ~ 670 nm, AD < 6 nm in diameter resolution can be achieved. Figure 1(b)

reveals no shift in MDR's positions along the ~ 2 mm fiber length. This suggests that the fiber is highly uniform over the estimated 2 mm fiber axis.

74
0>)

20-

- 15-

10-

5-

W

2

^-72.0 <SV2.5 <573.0 <S-73.5 <SV4.0

Figure 1 Elastic-scattering spectra along the fiber axis measured by detector at the shadow side

of the edge-illuminated fiber (a) side-view (b) front view.

The quasi-periodic spacing between MDR's of similar linewidth also allow absolute

measurements of average fiber diameter by matching the measured MDR's spacing and resonant

wavelength with the numerical spectra from Mie calculations.

The ripples between the series of sharp peaks might be due to unavoidable interference

from optical components in the optical path of the illumination and detection system. FabryPerot resonances were not pronounced because the fiber was transversely displaced from the

focused beam by ~ 60 |im such that the illumination at the fiber edge allows preferential light

coupling to the MDR's. For the data shown in Figs. 1(a) and (b), the detector and the focused

beam were on opposite sides of the displaced fiber.

For the spectra shown in Figs. 2(a) and (b), the detector and the focused beam were on the

same side of the transversely displaced fiber. The measured scattered light intensity is weaker

than the scattering intensity observed with the detector on the shadow side of the edgeilluminated fiber. Similar series of five MDR's were observed with no shift among the MDR's

along the estimated 2 mm fiber length. The broad resonances (with ~ 0.5 nm linewidth) are

apparently Fabry-Perot resonances, which allows ~ 0.1 |am resolution in relative fiber diameter

determination. The broad Fabry-Perot like resonances have a wavelength shift along the fiberaxis. We hypothesize that this wavelength shift might be related to slight fiber tilting in the

plane perpendicular to the incident beam. Further experimental investigations are required. In

fact, effects of fiber tilt in the incident-beam-fiber plane was recently studied using a similar

MDR's related experimental setup.^

75
(a) <T>)

15-

Figure 2 Elastic-scattering spectra along the fiber axis measured by detector at the illuminated

side of the edge-illuminated fiber (a) side-view (b) front-view.

In summary, we have a technique that can detect nm fluctuation of the cladding diameter

along mm length of the fiber axis. The absence of wavelength shift of narrow linewidth MDR's

suggests a uniform fiber diameter (less than 6 nm) along the fiber length of several mm ( ~ 2

mm).

We gratefully acknowledge the financial support from the National Science Foundation

under contract PHY-96 12200 and the helpful discussions with Dr. Daniel Nolan of Coming.

References

[1.] D. H. Smithgall, L. S. Watkins, and R. E. Frazee, Jr., Appl. Opt. 16, 2395 (1977).

[2.] J. F. Owen, P. W. Barber, B. J. Messinger, and R. K. Chang, Opt. Lett. 6, 272 (1981).

[3.] A. Ashkin, J. M. Dziedzic, and R. H. Stolen, Appl. Opt. 20, 2299 (1981).

[4.] A. W. Poon, R. K. Chang, and J. A. Lock, Opt. Lett. ( to be published ).

76
Measurement of the impact of fiber nonlinearities on

high data rate, dispersion-managed WDM systems

(invited paper)

Sebastien Bigo and Michel W. Chbat

Alcatel Corporate Research Center, Route de Nozay, 91460 Marcoussis, France

Phone: +33 1 69 63 14 78 / Fax: +33 1 69 63 18 65

E-mail: bigo@aar.alcatel-alsthom.fr

Abstract:

To upgrade high data-rate WDM transrriission systems, fiber nonlinearities must be coped with. This requires the knowledge of their strength, via, in particular, the nonlinear index coefficient Oj. We show some

measurement results of through four-wave mixing over several fiber types. Then, we measure the system

impact at 10 Gbit/s of various physical effects resulting from fiber nonlinearities : self-phase modulation (SPM),

cross-phase modulation (XPM) and stimulated Raman scattering (SRS). However, evaluating the impact of the combination of those effects is not straightforward, but it is essential for system design, especially dispersion

management, for which we show experimentally that residual dispersion can no longer be used as performance

criterion.

Introduction:

Two approaches are considered to upgrade WDM amplified transmission systems and thereby meet the ever growing demand for higher capacities on a single fiber. The first approach is to increase the bit-rate per channel. Simple optical noise considerations show that the signal-to-noise ratio (SNR), and thus the power

per channel, must be increased in the same proportion. In a typical 500-km-long link consisting of five spans

of realistic 28 dB power budget, the power requirement is such that, even at minimal recommended SNR (no

margin), the transmission can no longer be viewed as linear. The second approach is to increase the number

of channels, either by reducing the spectral spacing between channels or by occupying a larger amplifier

bandwidth. In both cases, the optical interactions between channels stemming from fiber cross-nonlinearities Increase significantly.

Therefore, to assess the performance of most high data-rate transmission systems, nonlinear effects must be

considered [1]. A list of them is given in the first part of this paper. Considering the variety of fiber infrastructures, it is of primary Importance to measure the strength of nonlinearities over each fiber type, and

In particular the nonlinear Index coefficient nj. We review the measurement methods that have been

proposed for measuring r\-2, and we discuss the limits of the four-wave mixing method [2] with all fiber types.

Then, we Investigate experimentally the system impact of the physical effects resulting from fiber nonlinearities at lOGblt/s : self-phase modulation (SPM), cross-phase modulation (XPM) and stimulated

Roman scattering (SRS). Unfortunately, their individual contributions to the system penalty cannot be added.

Thus, to Improve the design of a system, a simulation tool is highly recommended, especially for dispersion

management. Indeed, we demonstrate that the analytical rules yielding the acceptable residual dispersion at receiver end con no longer be applied at 10 Gblt/s.

Nonlinear effects in fibers:

Kerr nonllnearlty refers to the linear dependence of the fiber's refractive index on the injected channel power.

Silica, the major constituent of fibers, has one of the lowest nonlinearity coefficients (n2) of any optical

material. Nevertheless, Kerr-induced signal distortions can occur at reasonable powers (a few dBm) in the

77
fiber because of the large distances traveled and the energy confinement. This confinement is characterized

by the effective area of the fiber A^^. The smaller the higher the nonlinearities. (A^h = 80 jUm^ for standard single mode G.652 fiber (SMF), A^ff = 50 fjm^ for dispersion-shifted G.653 fiber (DSF)).

Various, generally detrimental, effects result from Kerr nonlinearities. In the first effect, referred to SPM, the

optical phase of a channel is modulated proportionally to its own instantaneous power. SPM appears as a broadening of the channel spectrum, as new optical frequencies are generated. However, since this effect primarily affects the signal phase, it does not affect the detection of intensify at the receiver end, unless the group-velocity dispersion (GVD) is non-zero. Indeed, the interplay of SPM results in a complicated phase-tointensity conversion process during propagation. Depending on GVD, SPM can be either beneficial - leading

to pulse compression - or detrimental - possibly leading to irretrievable breakup of the optical pulses.

The second effect stemming from Kerr nonlinearities is the nonlinear interaction of a signal with the amplified spontaneous emission (ASE) noise of the optical amplifiers, altering the SNR at fiber output. The

effect is less detrimental in the normal dispersion regime than in the anomalous dispersion regime, where it

is referred to as modulation instability (Ml).

In WDhA systems specifically, Kerr nonlinearities induce additional limitations such as the generation of intermodulation products between the carriers of a multiplex, a phenomenon called Four Wave Mixing

(FWM). In a nonlinear fiber carrying two or more Vv'DM channels, FV/M can create additional frequencies,

provided that a certain phase-matching condition is met, essentially if the relative velocity between the channels is low enough. Assuming the V/DM channels are equally spaced, the wavelengths of the

intermodulation products coincide with the other channel wavelengths, thereby creating undesirable in-band

crosstalk.

Another nonlinear effect to be considered is XPM. Like SPM, this phenomenon manifests itself as an

alteration of the optical phase of a channel, which translates into intensity distortions through GVD. However,

in contrast to SPM, it is a multi-channel effect, whereby the phase modulation of a channel is induced by the

intensify of the signal on neighboring channel(s). Thus it depends on the relative speeds of the interacting

channels, that is, on GVD. The lower the GVD (and thus the difference in channel speeds), the higher the

interaction between channels. However, at zero GVD, no phase-to-intensity conversion occurs. Consequently, it is not a simple matter to predict the impact of XPM and the other Kerr-related phenomena

and a simulation tool is often useful, as illustrated further on.

Finally, two additional nonlinear limiting effects can be encountered in transmission systems, stimulated

Brillouin scattering (SBS) and SRS. They do not originate from Kerr effect, but from a resonant parametric

interaction with acoustical and optical phonons, respectively, within the fiber medium. SBS has been

observed for a long time and well-known techniques can be implemented to reduce its impact. Conversely,

SRS is inherently WDM-related and its impact is expected to grow as the amplifier bandwidth increases, in order to accomodate new channels. When two channels located a few tens of nanometers apart propagate

along with each other, the lower-wavelength channel experiences power depletion through SRS, while

serving as a pumping signal to amplify the higher-wavelength channel.

All of these nonlinear phenomena can result in severe transmission impairments especially at 10 Gbit/s base

rate and above. It is therefore essential to be able to predict their impact over all existing fiber types, i.e. SMF, DSF, non-zero DSF (NZDSF) and dispersion-compensating fiber (DCF).

Measurement of nonlinear index nj through four-wave mixing:

The nonlinear index coefficient is one key parameter that is needed to characterize the strength of the Kerr

nonlinearify. All of the techniques that have been proposed for measuring n2 provide indirect estimation of nj

through one of the aforementioned Kerr-related phenomena. A large number of them resort to the measurement of the nonlinear phase, be it the result of SPM [3-6] or XPM [6-8]. These techniques use

interferometric devices, which are subject to environmental instability, a potential cause for large

78
uncertainties. In the other approaches, the distortions caused by SPM are analyzed through spectral

measurements [9-11], which requires a good knowledge of the initial waveform. The simplest timedependent waveform is probably a wave of sinusoidal envelope. Such a wave can be generated through the beating of two continuous-wave (cw) lasers, as in [1 1]. However, in this case, measuring the relative power

of the harmonic components induced by SPM amounts to measuring the relative power of the

intermodulation products induced by FWM as two cw tones propagate, a method described earlier in ref. [2]. The nonlinear index can be derived from these power measurements through an analytical formula in

[1 1]. In ref. [2], a better accuracy is obtained through numerical simulations. Only DSF was tested this way,

but one key advantage of the approach is that it can be extended to any fiber type with the same apparatus.

We apply next this last technique to three fiber samples and discuss its limitations.

In our setup, we use a DFB laser (1554.0 nm) and an external cavity laser to generate two continuous waves

of wavelengths separated by AX. The two waves, of equal power Pi^, are sent to two booster amplifiers of 20 dBm saturation power, combined and sent to the fiber under test. The polarizations of the waves are

ajusted until linear and parallel to each other using polarization controllers and a polarizer. After propagation, the signal is fed to an optical spectrum analyzer of 0.05 nm resolution. Typical input and

output spectra are shown in Figure 1. The index coefficient n-2 is recovered when the experimental power

ratio l,/lo between the carriers and the harmonics matches that obtained after several numerical integrations

of the nonlinear Schrodinger equation with a variable nj. To assert a good accuracy, the fiber length and the

spectral spacing AX must be carefully chosen. Indeed, as shown in Fig. 1, when AX increases, the efficiency of FWM and thus the ratio l,/lo decrease, then rapid pseudo-oscillations occur as a result of the break-up of the phase-matching condition. These rapid variations must be avoided because a small error during the measurement of AX would cause a potentially large error in the computed ratio l,/lo, leading to a wrong

estimation of nj. As illustrated in Fig. 1, this effect is all the more detrimental as the absolute value of the dispersion is high (up to approximately 80 ps/nm.km for the DCF). To circumvent it, small spacings AX or small fiber lengths must be used. However, AX cannot be made lower than 0.35 nm with our spectrum

analyzer. Moreover, reducing the fiber length not only eases the constraints on AX but also reduces the FWM

efficiency and the ratio Ii/Iq, which must remain significantly higher than the amplifier noise level for a good

accuracy. With the most troublesome fiber (DCF), we estimate the error on rij/A^ff to be ±10%.

20 ?CO•a

10

0

iwer

0.o

-10

-20

-30 J M . 1 1 — 1553 1554 1555 1556

20 - ?ea 10 •

0

-

'o

•

i

-10 • o

Q. -20 • '— 'l -30 - -.—1—1—,—.—,—rn T . -.—1 1 1553 1554 1555 1556 Wavelength (nm)

20 -r

0

r

>o -20

-40

-60

DSF (-0.2 ps/nm.km)

DCF (-72'ps/nm.km)

I I I I II I I I II I I

SMF

-t- 1 7 ps/nm.km)

NZDSF ( + 2.3 ps/nm.km)

0 0.5 1 1.5 2 2.5

Spectral spacing AX (nm)

FiQure 1 Principle of measurement using FWM (leff) and simulated effect of spectral spacing on hiarmonic-to-carrier ratio li/lg for various types of 1 -km long typical commercially available fibers fed by two cw tones atl 7 dBm. The results are summarized below (all parameters measured at 1550 nm).

Dispersion Length loss n2/Aeff "2

(ps/nm.km) (km) (dB/km) (10'°/W) (/7m^) (10-'° m'/W)

DCF -81.3 0.50 0.54 7.20 35 2.5

SMF 17.7 1.00 0.20 3.20 78 2.5

NZDSF -2.09 2.00 0.22 5.62 48 2.7

79
The knowledge of the nonlinear strength, through nj measurement in particular, is one unavoidable step

towards the assessment of the impact of fiber nonlinearities in transmission systems. Another step is the characterization of all the phenomena that result from them.

Measurement of system impact of nonlinearities:

We now report an investigation of the limitations induced by SPM at 10 Gb/s over four types of fiber infrastructures [12]: DSF, (-0.1 ps/nm.km [b] and +0.1 ps/nm.km [c]), NZ-DSF with negative

(-2.3 ps/nm.km [a]) or positive (-1-3 ps/nm.km [d]) GVD, and SMF (17 ps/nm.km [e]) followed by 10km of dispersion-compensated fiber (DCF, -80 ps/nm.km) to reduce the impact of GVD. The 2x100 km-long link (2x90 km in [e]), is fed by a transmitter incorporating a 10 Gb/s chirp-free intensity modulator. The

transmission performance is characterized by the power penalty at 10''° bit-error rate (BER). Insight on the

origin of this penalty is given by the geometrical eye-diagram, computed from time-averaged waveforms,

free of the noise contribution. The power P at fiber input is varied within [2 dBm, 20 dSm].

The experimental results are represented in Fig. 2(a). In all configurations, a BER floor is measured within the

investigated power range: a 3 dB penalty being reached at 8, 16, 16, 12, 1 7dBm in coses [a] to [e],

respectively. At high GVD ( bl >2ps/nm.km), the geometrical eye-diagram shows that the predominant

detrimental effect is waveform distortion. For negative GVD [a], the interplay between GVD and SPM

broadens isolated pulses, but does not improve the transmission. For positive GVD [c, e], higher-order

soliton-like effects increase the extinction ratio and hence reduces the penalty.

NZ-DSF DSF

2,3 ps/nm.km) (+0,1 ps/nm.km

/ NZ-DSF /p -0,1 ps.nm.km)

• " (+3 ps/nm.km)

10 15

Input power (dBm)

Figure 2- Effect of SPM : power penalty at 10 '° BER as a function of input power

However, above a certain threshold

(P=10 and 17 dBm for [b] and [e],

respectively), the penalty sharply

increases as an irretrievable breakup of pulses occurs. V/ith DSF [b, c], the power penalty is almost zero until P reaches 16 dBm. Above that threshold, signal-ASE nonlinear

interactions cause a dramatic

reduction of SNR.

Simulation results (Fig. 2(b)) with the

above parameters are in good _ agreement with the experiments,

,,^.,-0.1ps.nm.km) ^^^^^^ ^^3^ ^.^^

power levels: the contribution of noise is computed analytically and

added at the receiver end, which

overlooks noise-signal interactions during propagation.

DSF

(+0,1 ps/nm.km

Consider now nonlinearities involving several channels. In the following, we investigate the impact of XPM

using Q very similar apparatus as that used for SPM. Two independently-modulated cw lasers, referred to as

the probe and the pump, are combined, boosted and sent to two 100-km-long spans of NZDSF

(-1-3 ps/nm.km) or DSF (4-0.1 ps/nm.km). The power of the probe ( + 2dBm) is maintained 6 dB lower than

that of the pump so that SPM effects to the probe can be neglected. SPM effects to the pump are assumed

second-order effects. The polarization and the rf delay between the two waves are adjusted manually for best and worst BER.

80
The transmission performance is characterized through the power

penalty at 10-'° BER. The results are

displayed in fig. 3. When the channel

decreases down to 100 GHz, the worst case penalty increases

dramatically by 1 dB over DSF and by

2.2 dB over NZDSF. Conversely, the

best case penalty is almost unchanged

and corresponds to the penalty

obtained in the linear regime. This

result shows that, at 8 dBm and with

only two channels, XPM can almost be

suppressed by proper polarization

arrangement at transmitter output.

Should this arrangement not be

carried out, the pump power or the

number of channels be higher, the

impact of XPM alone was found highly

detrimental.

worst cose

+ 3 ps/nm.km

100 200 300

Channel spacing (GHz)

400

Figure 3. Effect of XPM : power penalty at 70 '° BER of as a function of power at fiber input.

Another WDM nonlinear effect is SRS. We characterize its impact in an unrepeatered 100-km long

transmission based successively on two types of NZDSF ( + 2 ps/nm.km, and -2.3 ps/nm.km in average)

employing 32 channels at 10 Gbit/s. At transmitter output, the power per channel is 5 dBm with a very low

power excursion between channels (<1 dB). After propagation, a spectral analysis shows that the lowerwavelength channels have lower power levels than the higher-wavelength channels. Channel #1 to channel

#32 relative loss difference is 2.6 dB, whereas the linear loss dependence of the fiber on wavelength leads

to less than 1 .5 dB only. The additional loss on lower-wavelength channels can be attributed to SRS. Our

simple analytical model of SRS yields results in very good accordance with the experiments.

Impact of nonlinearities on dispersion management:

The experimental results described above show how severe nonlinear impairments can be at 10 Gbit/s.

These impairments depend strongly on the fiber dispersion, as a result of the interplay with GVD. This

explains why compensating for too much cumulated dispersion along the link, i.e. managing the dispersion,

should be done carefully. Indeed, as a result of nonlinearities, the range of acceptable residual dispersions

at receiver end is significantly reduced with respect to the commonly-used [-1000, -1-1000 ps/nm] range in the linear regime. This explains why, contrary to what was originally thought, dispersion management is compulsory not only in terrestrial links using SMF, but also in those using low dispersion NZDSF [1 3].

Alternate NZDSF Alternate NZDSF NZDSF

( + 3.0 / -2.7 ps/nm.km) (-1-3.0 / -2.7 ps/nm.km) (-1-3 ps/nm.km)

0 200 400 600 0 200 400 600 0 200 400 600 Length (km) Length (km) Length (km)

Figure 4 . Experimental penalties (inset) a 10 '° BER of every other channel in 8x10 Gbit/s transmission over three 500 km-long

dispersion maps chargcterized by the same residual dispersions. The gray area corresponds to error floors.

81
Another consequence is that, at 10 Gbit/s, residual dispersion cannot serve as a performance criterion. To

illustrate this point, eight 200 GHz-spaced channels at 10 Gbit/s have been transmitted over five 100 km

long spans with a realistic 28 dB loss per span. Five flat-gain, dual stage silica/fluoride amplifiers provide

5 dBm power per channel, yielding 23 dB SNR (in 0.1 nm bandwidth) at the end of the link. The transmission

fiber consisted either of NZDSF with 3 ps/nm.km dispersion or alternate sections of NZDSF with 3 ps/nm.km

and with -2.7 ps/nm.km dispersions. GVD compensation was achieved through DCF or SMF in the terminals

and in-line when needed, but in the three investigated configurations, the residual dispersion is the same

(350 ps/nm) for the multiplex central wavelength. The dispersion maps and the measured power penalties of every other channels are represented in Figure 4. It can be seen that the performance depends a lot on the transmission fiber, and, with the same transmission fiber, on the arrangement the dispersion compensating

units in the terminals. Clearly, residual dispersion is not sufficient to assess the performance of a system.

Discussion and conclusion:

We have carried out experimental investigations of the limits induced by SPM, XPM and SRS individually in order to estimate their relative impact over several types of infrastructures. However, when all these

phenomena are mixed altogether, as in an actual WDM transmission, a major difficulty is that the system

power penalty is not just the sum of the individual penalties. Therefore, simulation tools are strongly

recommended not only for performance evaluation but also for system design. In particular, we have

conducted a comprehensive numerical investigation of dispersion management over several types of fiber infrastructures which coincide very well with the experimental results presented in this paper [14]. However,

simulation tools require the characteristics of the fiber be accurately known, in particular the nonlinear index

coefficient nj, for which some measurement techniques have been shown.

Acknowledgement: The authors are grateful to Alain Bertaina and Gaelle Le Meur for their contributions

to this work.

References:

[1] A. Chraplyvy, "Limitations on lightwave communications imposed by optical fiber nonlinearities ", IEEE J. Lightwave

Technol., vol. 8, pp. 1548-1557 (1990)

[2] L. Prigent et J. -P. Hamoide, "Measurement of fiber nonlinear Kerr coefficient by four-wave mixing", IEEE Photon.

Technol. Lett., vol. 5, pp. 1092-1095 (1993)

[3] A. J. Taylor and G. Rodriguez, "Determination of by direct measurement of the optical phase", Optics Lett., vol. 21, n°22, pp. 1812-1814 (1996)

[4] L. P. Barry, J. M. Dudley, P. G. Bollond, J. D. Harvey and R. Leonhardt, "Simultaneous measurement of nonlinearity

and dispersion using frequency resolved optical gating", Electron. Lett., vol. 33, pp. 707-708 (1997)

[5] M. Artiglio, R. Caponi, F. Cisternino, C. Naddeo, and D. Roccato, "A new method for the measurement of the nonlineor refractive index in optical fiber". Optical Fiber Technol., vol 2, pp. 75-79

[6] A. Fellegara, M. Artiglio, S. B. Andreasen, A. Melloni, F. P. Espunes, and S. Wabnitz, "COST 241 intercomparison of nonlinear refractive index measurements in dispersion shifted optical fibers at X=1550 nm". Electron. Lett., vol. 33,

n°13, pp. 1 168-1 169 (1997).

[7] T. Kato, Y. Suetsugu, M. Takagi, E. Sasaoka, and M. Nishimura, "Measurement of the nonlinear refractive index in optical fiber by the cross-phase modulation method with depolarized pump light". Optics Lett., vol. 20, n°9, pp. 988-

990 (1995)

[8] L. Amoto, A. Fellagara, P. Sacchatto, P. Boffi, A. Melloni, and M. Martinelli, "Polarization-independent Kerr

coefficient measurement in optical fibers". Optics Lett., vol. 21, n°8, pp. 612-614 (1996)

[9] K. S. Kim, R. H. Stolen, W. A. Reed and K. W. Quoi, "Measurement of the nonlinear index of silica-core and

dispersion-shifted fibres", Optics Lett., vol. 5, pp. 257-259 (1994)

[10] Y. Namihira, A. Miyata and N. Tanahashi, "Nonlinear coefficient measurements for dispersion shifted fibres using

self-phase modulation at 1 .55 jum", Electron. Lett., vol. 30, pp. 1 1 71 -1 1 72 (1 994)

[1 1] A. Boskovic, S. V. Chernikov, J. R. Taylor, L. Gruner-Nielsen and O. Levring, "Direct continuous-wave measurement

of n-2 in various types of telecommunication fiber at 1 .55 ^m", Optics Lett., vol. 21 , pp. 1 966-1 968 (1 996)

[12] S. Bigo, S. Penninckx, M. W. Chbat , "Investigation of self-phase modulation limitation on 10 Gbit/s transmission

over different types of fiber". Optical Fiber Communications Conference, paper FC2, San Jose, 22-27 Feb 1998.

[13] A. Bertaina, S. Bigo and M. W. Chbat, "Investigation of the limitations of WDM typical terrestrial transmissions over NZDSF and SMF", European Conference on Optical Communications, TuCl 1, Madrid, 20-24 Sep. 1998.

[14] A, Bertaina et al., to be submitted

82
Highly Accurate Nonlinear Coefficient Measurements by SPM

Method for DSFs and Large Effective Area Fibers at 1.55jim

Yoshinori NAMIHIRA

KDD R&D Laboratories Inc.

2-1-15 Ohara, Kamifukuoka, Saitama 356-8502, Japan

Phone: +81-492-78-7801, FAX: +81-492-63-9328, E-mail: namihira@lab.kdd.co.jp

Introduction: Optical nonlinearites limit the maximum transmitted power through

optical fibers and have become a serious problem in ultra-long distance transmission

systems using Er-doped fiber amplifiers (EDFAs) such as transoceanic optical fiber

undersea cable systems. Therefore, accurate determination of the nonlinear coefficient

{n2/Aejf; n: is the nonlinear refaractive index, Aejfis the effective area [1]) of optical

fibers is required for the amplifiered optical transmission systems. To date, the

nonlinear coefficient of the optical fibers have been measured by using the self-phase

modulation (SPM) method [2-5], the cross-phase modulation (XPM) method [6] and the

four-wave mixing (FWM) method [7,8] at 1.55|im region.

This paper presents effect of chromatic dispersion in fiber length (DL) on the measurement accuracy of the nonlinaer coefficient and nonlinear refractive index

measurements by the SPM method for the dispersion shifted fibers (DSFs) and the large

effective area fibers (LEFs) for the first time.

Experiment: The experimental set-up of the nonlinear coefficient (n2/Aejf)

measurements for the various DSFs and the LEFs by the SPM method is shown in Fig. 1 [5]. Wavelength tunable transform limitted (TL)-optical short pulse with variable

repitition rates was generated by using a tunable CW laser source, a sinusoidally

modulated InGaAsP electroabsorption (EA) modulator (JAE, FOEA-lOOB-023) [9], and a LiNb03 (LN) modulator. At first, -650 MHz optical pulse train with pulse duration of

200 ps was generated by using the LN modulator driven by a pulse-pattern generator.

To reduce the pulse width, the optical signal was modulated again by the EA modulator

driven by 5 GHz sinusoidal wave [5]. The wavelength range was 1545-1565nm which was limited by the EDFA Gain bandwidth. The optical pulse width (FWHM, Ax) of

around 21 .7 ps was meausred by the autocorrelator (Oyokoden LAB, FEMTOWAVE Model

775) and the oscilloscope (HP 54120A). The autocorrelator trace of the obtained input

optical short pulse is shown in Fig. 2 [5]. From Fig. 2, it was found that the optical

pulse shape was almost Gaussian. The typical peak power Po of the optical pulse was

around 1 W [5]. The spectral bandwidth Av was around 20.7 GHz. Accordingly, the

time-bandwidth product of At and Av was arround At - Av 2 0.45, which means the optical

pulse shape was very close to TL-Gaussian pulse. The output optical pulse was

measured by the optical spectrum analyzer. As the input optical power increases, the

maximum phase shift ({)max increases in proportion to the input peak power Po, eventually

wavelength spectra broadenes as shown in Fig. 3. Fig. 3 shows the output frequency

spectra of the LEF of 6km long at (I)max=3.57i. The (n2/AeJf) was obtained from the

relationship between the 0max and Po. The n2 can be estimated from the (nl/Aeff)

multiplied by the Aeff. The mode field diameter (MFD) was measured by the variable

aperture in the far field MFD measuring equipment (PK Technol. S25) [1]. From the far

field pattern (FFP) of the output optical power E(r)^ it is then possible to calculate the near field pattern (NFP) using Hankel transformation [1]. Accordingly, the Aej^can be

derived from the NFP, E(r)- converted from the FFP. Here, AeffwsLS measured by the

MFD measuring equipment with the software of NFP converted from FFP [ 1 ].

Results and Discussion: In the experiments, various DSFs and LEFs were

measured, but the refractive index profiles (RIPs) were quite different. Here, DSFs 1,2,

and 4 are dual-shape core (DSC) RIP type as shown in Fig. 4. Meanwhile, LEFs-2, 3 and

83
4 are segmented core (SGC) RIP type, and LEFl is depressed segmented core (D-SGC)

RIP type, respectively. In addition, LEF5 is special one, and it has Germanium (Ge)

doped center core, and Fluorine (F) doped side (second) core and depressed cladding (FDC) RIP.

Figs. 5 (a) and (b) show {n2/AeJf) and n2 vs. chrmatic dispersion in fiber length (DL) of

various DSFs and LEFs. Here, DL is given by eq.( 1 ).

DL= - So (Xo - X) L (1)

where, Xo is zero-dispersion wavelength. So is the dispersion slope at Xo, X is the

measurment wavelength, and L is the fiber length.

From Figs. 5 (a) and (b), it was found that the DL becomes larger, the values of

(nl/Aeff) and n2 become smaller. It can be seen from Figs. 5 (a) and (b), the

experimental results of (nl/Aeff) and n2 are under estimated when the chromatic

dispersion is large . Accordingly, it was confirmed that the DL limits of {nllAeff} and n2

measurement s for optical fibers were found to be less than -2.0 [ps/nm] at Region A for

DSFs, and -8.0 [ps/nm] at region B for LEFs.

Figs. 6 (a) and (b) show the SPM spectra of LEF2(SGC) at Point C (DL= -7.9 [ps/nm])

and Point D (DL= -28.4 [ps/nm]) as shown in Figs. 5(a) and (b). It can be seen from

Figs. 6 (a) and (b) that there are great discrepancy of two SPM frequency spectra at

(|)max=2.57t of the same LEF2(SGC) because of dispersion (DL) effects.

By considering DL limits, the nonlinear coefficient measurement results for various

DSFs and LEFs at 1545-1565nm regions are summarized in Table 1 and Table 2,

respectively. From Table 1, the averaged values of (nl/Aeff) and n2 for various DSFs

were found to be around 4.86 x

10"'° [1/W] and 2.25 x 10 -° [m'/W] , respectively.

Also, from Table 2, it was found that the averaged values of (nl/Aeff) and n2 for various

LEFs are around 2.49 (2.61) x

10"'° [1/W] and 2.00 (2.07) x lO"'" [m-/W] , respectively.

Here, ( ) means the averaged value obtained from without using LEF5(F-DC) data.

The values of 2.25 x

10"'° [m'AV] of DSFs and 2.00 (2.07) x10"'° [m'AV] of n2 for

various LEFs for random polarization states are in agreement with that of 2.1 - 2.3 x

10'^° [m^/W] range of recently published results of the SPM [3-5], XPM [6] and FWM [7,8]

methods, respectively.

Conclusion: By considering chromatic dispersion effect, a highly accurate nonlinear coefficient (nl/Aeff) and nonlinear refractive index n: measurements in SPM at

1 .55 jim was performed for the first time. It was clarified that the chromatic dispersion

limits of (nl/Aeff) and n2 measurements were found to be less than -2.0 [ps/nm] for

dispersion shifted fibers, and -8.0 [ps/nm] for large effective area fibers.

Acknowledgment: The author would like to thank M. Mure, Y. Tanaka of

Oyokoden LAB. Co. Ltd., and M. Suzuki, I. Morita, H. Taga and Y. Matsushima of

KDD R&D Labs, for their experimental supports, and also thank T. Muratani, K. Suzuki,

Y. Mimura, and S. Akiba of KDD R&D Labs., for thier encouragement.

References:

[1] Y. Namihira, Electron. Lett., 30, 3, pp. 262-263, 1994.

[2] K. S. Kim.etal., Optics Letters, vol. 19, no. 14, pp. 257-259, 1994.

[3] Y. Namihira etal.,Electron. Lett., 30, 14, pp.1 171-1 172, 1994.

[4] R. H. Storen etal., OFC'95., FD 1, pp.312-313, 1995.

[5] Y. Namihira, OFMC'97, NPL,UK, pp.50- 53, 1997.

[6] T. Kato, et al.. Optics Lett., 20, 9, pp. 988-990, 1995.

[7] L. Prigent, etal., IEEE, Photonics. Technol. Lett., 5, 9, pp. 1092- 1095, 1993.

[8] A. Boskovic, et al.. Optics Lett., 2 1 , 24, pp. 988-990, 1 996.

[9] M. Suzuki etal., IEEE, JLT, vol.1 1, no.3,pp.468-473, 1993.

84
Wa\«length Tunable

Transform Limited

Optical Pulse Source

with External Modulators

OBPF

- 1 nm

OBPF

- 1 nm

X = 1545 - 1565 nm

/m = 650MHz

Ax H 2 1.7 ps

Av =20.7 GHz

OBPF: Optical Band Pass FUter EDFA: Er-Doped Fiber Anplifier At Av = 0.45

0PM

Optical

Spectrum

Analyzer

(OSA)

Optical

Power

Meter (OPM)

Autocorrelator

or

Oscillo Scope

(OSC)

Fig.l Experimental setup for nonlinear coefficient measurements of optical fibers using

tunable transform-limited optical short pulse source with external modulators

Reenter @1554nm

III II M

II I II II

—n—I 11 I I II I M 1

1

—ii4_u_

3.5 n

-30 -24 -18 -12 -6 0 24 30 Time [ps]

Fig. 2 Input optical Transform-limited Gaussian

pulse shape (measured by Autocorrelator)

Wavelength k [nm] ^ ^ Frequency v [GHz]

Fig. 3 Output SPM frequency

spectra of the LEF (L=6km)

[@(|)max=3.57r]

Ge-Doped

.t 0

Dual Shape Core

[DSC]

DSF : Dispersion

Shifted Fiber

Segmented Core Depressed-SGC

[SGC] [D-SGC]

Fluorine DopedDepressed Core

[F-DC]

LEF : Large Effective Area DSF

Fig.4

F-Doped

Various refractive index profiles (RIPs) of Dual-Shape Core (DSC),

SegmentedCore (SGC), Depressed-Segmented Core(D-SGC), and

Fluorine doped-DepressedCladding (F-DC)

85
I I I I I I I I I I I I I 1 I I I I . .

Chromatic Disperaon in Fiber Length DL (ps/nm]

-25 -20 -15 -10 -5.0 0.0 Chromatic Dispersion in Fiber Length DL [ps/nm]

(a) nillKejf vs. DL= - So (ko - \) L (b) 02 vs. DL=- So (Kq - X) L

Fig. 5 nllAejf {di) and n2 (b) mesurements vs. chromatic dispersion in fiber length (DL)

Table

LEF2 (SCO I 1 U 1 u

DL= -7.90

1 ps/nm 1

i p. A 11

1 (a) Point C

i 1 '!

! 1 i

1 nil i i

1

a a J :: A tcnlcr

!i " ii

lOnm/lliv

(! ii

l,FK2

I SCO ( ps/nm I

I . i i 'Wj\\—

T Jhnji = 2.5 K

1 Wavelength A. |nm] ^ ^ Frequency V [GHz I

\Vavelen{;th X jnmj ^

^ Frequency v ICHzj

Fig. 6 SPM frequency spectra of LEF2 at Point C (DL=-7.9[ps/nm])

(a) and Point D (DLs -28.4[ps/nm]) (b)

Nonlinear coefficient measurements results for various DSFs at 1545-1565 nm

DSFs

(») DL

[ps/nm)

\eff

(UnT ) @X

mlXeff

10

[10 /W] @X

n2

-20 2 [10 m /W) @X

DSFl

(DSC) - 1.29 47.3 4.90 2.32

DSF2

(DSC)

- 0.99 43.7 5.11 2.23

DSF3

(DSC)

- 1.66 46.6 4.99 2.21

(*) DL= - S)(Xo- X)L Average 4.86 2.25

Table 2 Nonlinear coefficient measurements results for various LEFs at 1545-1565 nm

LEFs

(•) DL

[ps/nm) [pm ') @X

nJAe//

-HI [10 rw] @x

n2

.211 2 (10 m AV @x

I.FFl (D-SCC)

-6.87 75.6 2.71 2.05

l,F.F2

1 SCO -5.54 75J 277 209

I.FF.I

( SCO -7.90 76.7 2.69 2.06

I.FF4

I SCO -3.34 92.0 2.26 208

I.FFS (#)

( F-DO - 777 85.0 2.00 1.70 '*

(•) DL= - SiiXo- X)L

( #) Fluorine Dtiped Cnre

Average

( wit hout #

249

( 2.6 1)

2.00

(2/)7)

86
Monitoring of optical signal quality

using sum-frequency-generation optical sampling

Ippei Shake. Hidehiko Takara, Satoki Kawanishi and Yoshiaki Yamabayashi

NTT Optical Network Systems Laboratories

1-1 Hikari-no-oka, Yokosuka, Kanagawa, 239-0847 Japan

Introduction

One urgent problem to solve in realizing optical networks is monitoring the signal quality in

the transmission fiber. A comprehensive and precise monitoring method is needed to determine

failure location. Conventional digital networks commonly perform parity checking of the total

bit sequence at each regenerator repeater for this purpose. One benefit of optical networks is

their bit-rate or signal coding transparency [1], so the last measure we want to adopt is equipping line repeaters with bit-error monitoring or parity checking circuits at every

monitoring point. The transparency would be lost because digital monitoring is workable only

with predetermined bit-rates and frame formats. The new optical method proposed herein

makes it possible to monitor the signal quality or signal-to-noise ratio (SNR) of a digital signal

regardless of its bit-rate, frame format or even modulation format.

This method utilizes the sum-frequency-generation (SFG) optical sampling technique with

the temporal resolution of 1 ps [2], and measures amplitude histograms of optical signals ailoptically. Therefore, it can monitor the averaged quality (Q-factor) of optical signals in the

optical networks with any bit-rate or coding. We also demonstrate averaged Q-factor

monitoring of RZ or NRZ 10 Gbit/s optical signals using the SFG optical sampling method.

Principle

Figure 1 shows the fundamental configuration of the averaged Q-factor monitoring

technique. The optical signal is split from the transmission line and fed to the Q-factor monitor

system. Optical signal and optical sampling pulses are combined and injected into an SFG

crystal in order to generate the cross-correlation signals (the SF lights). These signals are

detected and the sampled values are processed to obtain an amplitude histogram.

Figure 2 shows the eye-diagram and the amplitude histogram. The amplitude histogram

exhibits amplitude distributions in both mark and space levels. The Q-factor {Q{to)) is estimated

from the amplitude histogram which is generally made at a fixed timing phase {tg) in the pattern

as opposed to the data eye [3]. It is defined by

Q{to) = I Hi (to) - fio(to) I / (oi (to) + oo(to) ) (1)

where /z/ (to) and <7/ (to) are the mean and standard deviation of the mark( 1 ) and space(O) levels

at to, respectively. Whereas in this report, we define the averaged Q-factor (Qave) by

87
,ave ,ave + CJO.ave ) (2)

where jii^ave and Oi^ave 'ire the mean and standard deviation of the mark(l) and space(O) levels

of all sampled data, respectively. However, the sampled data obtained by optical sampling

includes unwanted cross point data in the eye-diagram, which decrease the measured value of

the averaged Q-factor. Thus, it is necessary to remove the cross point data. In this report, we

set two threshold levels at /Jo+cciJ- and /dj-aji. The coefficient a is defined to lie between 0 and

0.5. We regard histograms whose amplitudes are larger (smaller) than ^i-a/n {jjio+aii) as mark

(space) level distributions. For both distributions, the means and standard deviations are determined. From these values, we can estimate the averaged Q-factor. Since this method

measures the averaged Q-factor, it fundamentally doesn't need a clock recovery circuit and we

can monitor the signal quality regardless of the bit-rate, frame format or even modulation

format.

Experimental Setup

The Q-factor monitoring system is based on the optical sampling system that was reported

previously [2]. We used either an NRZ optical signal or a RZ optical signal. The NRZ optical

signal was generated by DFB laser and LiNb03 intensity modulator. The RZ optical signal was

generated by a mode-locked Er-doped fiber laser and the same modulator. The wavelength Xsig

of both lasers was 1553 nm. The bit-rate /o of the optical signal was 10 Gbit/s. The pulse width

of RZ optical signal was 4.0 ps. The repetition rate of the sampling pulses was set to 9.7 MHz

(/o /1024 GHz ) minus the frequency offset 4/ of 10 kHz. The pulse width and wavelength of

the sampling pulse X^am were 0.41 ps and 1546 nm, respectively. The sampling pulses and

signal were combined by an optical coupler and injected into a SFG crystal. The crosscorrelation signals (the SF light) were detected with an APD and the sampled data was

processed by the signal processing circuit. We measured the averaged Q-factor while changing

the SNR using the optical attenuator placed before the Er-doped fiber amplifier in the

transmission line. We also measured the bit-error-rate (BER) of the signal in the transmission

line, and Q{to) is estimated by equation (3) when the amplitude distributions are fit to a

gaussian characteristic [3]

where erfc(;c) is a complimentary error function.

Results and discussion

Figure 3 shows the measured amplitude histograms of the NRZ optical signal. At high

SNR (Fig. 3(a)), both peaks of mark and space are found clearly. Whereas for low SNR

(Fig. 3(b)), the mark peak level becomes lower and Oj becomes larger due to ASE noise from

an Er-doped fiber amplifier in the transmission line. Figure 4 shows the relationship between

the averaged Q-factor and Q{to) calculated from measured BER for NRZ optical signal, when

BER = Qdcmo)) (3)

88
signal SNR and coefficient a were changed. We can see that the averaged Q-factor changes

with Q{to). The maximum value of Qito) in this experiment is 16.5 dB which corresponds to

the BER of 10'' ^ so we expect this method can be used when the BER is less than 10'",

According to Fig. 4, the optimum value of a is 0.3. In the cases of « < 0.2 and a > 0.4, the

values of measured Q-factor fluctuate. The numbers of sampled points are not enough to decide

the values of the mean and standard deviation in the case of a < 0.2. In case of a > 0.4, the

sampled cross points of eye-diagram are included into either mark or space level and the mean

and the standard deviation values are fluctuated. At a = 0.3, the slope SQave/^Qi^o) of linear

fitting was 0.68, and a very high linear correlation coefficient of 0.99 was obtained. These

results indicate that we can monitor signal quality by measuring the averaged Q-factor using

this method. We can also know the BER by measuring the averaged Q-factor and referring to Figure 4 and equation (3).

Figure 5 plots the relationship between the averaged Q-factor and Q(to) for the RZ optical

signal. When a is 0.3, the slope of linear fitting is 0.68 and the linear correlation coefficient is

0.99, so we can also use this method for RZ optical signals. The pulse width of the RZ optical

signal was 4.0 ps which is adequate for handling 100 Gbit/s optical signals, so this method can

be applied to 100 Gbit/s TDM signals.

Conclusion

We have proposed a novel method of monitoring optical signal quality. This method utilizes

a SFG optical sampling technique. We have confirmed that the NRZ or RZ optical signal

quality at the BER of 10'" can be estimated by this method, and that the averaged Q-factor

measured by this method shows a strong correlation to Q(to) calculated from the BER (the

linear correlation coefficient is 0.99). It is expected that this method can monitor the signal

quality of transmission systems at any bit-rate (up to 100 Gbit/s), any frame format, and even

any modulation format.

Acknowledgment

The authors would like to thank Dr. Ikutaro Kobayashi for his encouragement.

References

[1] Paul. E. Green, Jr., "Optical Networking Update", IEEE J. Select. Areas Commun.,

vol.14, No.5, pp. 764-779 (1996)

[2] H. Takara, S. Kawanishi, A. Yokoo, S. Tomaru, T. Kitoh and M. Saruwatari, "100 Gbit/s

optical signal eye-diagram measurement with optical sampling using organic nonlinear optical

crystal", Electron. Lett., vol.32, No.24, pp. 2256-2258 (1996)

[3] Neal S. Bergano, F. W. Kerfoot, and C. R. Davidson, "Margin Measurements in Optical

Amplifier Systems", IEEE Photonics Tech. Lett., vol.5, No.3, pp. 304-306 (1993)

89
Optical

laser source

Optical signal Coupler

Pulse repetition rate /q

Optical frequency oi^^^ r

ATT : attenuator mod. : modulator O/E : optical receiver

Transmission line O/E Error

detector

Q-factor monitor system

SFG APD

ciystal

Signal processing

circuit

Sampling optical pulse

Sampling optical pulse source

^ SF Ught

Amplitude

Fig. 1 Fundamental configui^tion for Q-factor monitoring

Amplitud^k

mark T 1 J

Number of times 0

Fig. 2 Eye-diagram and amplitude histograms

Time

1600

1400

1200

1000 800 600

400 200

0

NRZ

ikuuiMllld lllllllll...... Amplitude (arb. unit)

(a) highSNR (BER < lO"")

NRZ

liiilllll llllll ililihi.1.... Amplitude (arb. unit) (b)lowSNR (BER -10-5)

Fig.3 Amplitude iiistograms of the NRZ optical signal

12 13 14 15 17

(?(fo) (dB)

Fig. 4 Relationship between the averaged Q-factor and (?(fo) (NRZ optical signal) 22

tt 20

18 gl6

oI

14 6 ,2

"S^ 10 u

S 8 u^ 6

: O: a

1 •• ' • 1 '

-0.1

' ' 1' ' ' ' 1 1 ' ' 1 1 1 • 1 o : T X : a -0.2 RZ '. A

a -0.3

" : a - 0.4

' + : a -0.49 -X-

-X

ix-

. .-

A--

....4- •¥

'

1 1 i 1 1 1 1

1 . . . 1 ....

10 11 12 13 14 15 16 17 W(dB)

Fig. 5 Relationship between the averaged Q-factor and Q(tQ) (RZ optical signal)

90
High Resolution Reflectometry for Diagnosis of Optical Devices

by Synthesis of Optical Coherence Function

Kazuo Hotate and Takashi Saida

Department of Electronic Engineering. School of Engineering

The University of Tokyo

7-3-1 Kongo. Bunkyo-k'u, Tokyo 1 13-8656. Japan

1. INTRODUCTION

The optical frequency of a laser diode (LD) can be controlled by directly modulating the injection current.

In this way, many kinds of functional optical operations can be realized. For example, we can control the

linewidth. i.e., the coherence length of the source by modulating the injection current with white noise. We

have developed several oj)tical sensing systems using this method, such as optical fiber interferometric sensors with linewidth control,'' ' and a quasi-heterodyne optical fiber sensor.' Furthermore, We proposed and demonstrated that not only the coherence length but also the shape of the coherence function can be controlled and scanned by appropriately modulating the optical trequency of the

laser source and the ^hase diflference in an interferometer. We call the technique ''the synthesis of optical

coherence function." It can be understood physically that a frequency modulation of the laser source produces a correspondent power spectrum in the view of time averaging. Thus a particular shape of

coherence function, which is given as the Fourier transformation of the power spectrum,' is synthesized. The

technique has found various applications in optical reflectometry, distributed and multiplexed fiber-optic

sensing, and optical information processing with the advantages of neither post-calculations nor mechanical

scanning.

In this presentation, the principle of the synthesis of optical coherence function is summarized, and a

typical synthesized coherence function, delta-function-like coherence function, are given. The performance

deterioration factors and correspondent countermeasures are shortly discussed. Then, the reflectometry

systems for diagrosis of optical devices are shown.

2. SYNTHESIS OF OPTICAL COHERENCE FUNCTION

2.1 Principle of the synthesis of optical coherence function

6-13

Consider a modified Mach-Zehnder interferometer with a phase modulator at one of the two arms as shown in Fig. 1 . The visibility of the interference pattern observed at the screen is defined as

(1)

where /ma.x and 1,^ are the local maximum and minimum intensities, I\ and I2 the intensities from arm I and arm II. respectively, rthe differenfial time delay between the two arms, and \}{ r)| the optical coherence function.

In general, the optical coherence function is determined by the optical spectral characteristics of the light source. When we synchronously modulate the optical frequency of the laser source and the phase difference in stepwise waveforms of steps, the optical coherence function can be expressed as

(2) k„=t„exp (3)

wheref„. <f>„ and /„ are the optical frequency shift, the differential phase shift and the time duration at the wth

91
step of the modulations. Because Eq. (2) is in the form of a Fourier series with a complex coefficient k„, the coherence function }{ r) can be synthesized into any arbitrary shapes theoretically by modifying the parameters

/„, (p„ and /„ appropriately, i.e., by appropriate waveforms of the frequency and phase modulations. For

example, the modulations shown in Figs 1(a) and (b) will produce a coherence function of tnangle shape as shown in Fig. 1(c). In practice, we have synthesized coherence functions, such as a notch-shaped function, a square-shaped function and a sharp notch-shaped function.'''

'^

The coherence function can be scanned by the phase modulation. When the phase (/)„ is modulated in proportion to the optical frequency shift as ^ = IrfnK, Eq. (2) becomes

2r„exp[/-2;^„(r-x-)

Z'.

It can be seen that the coherence function is scanned by the delay of k.

One of the most useful synthesized coherence function may be that of the delta-function-like shape, which

has been applied for various distributed information detection systems. When the stepwise optical frequency

modulation is equally spaced as shown in Fig. 2(a), delta-function-hke peaks are synthesized, which is shown

in Fig.2(c). Additionally we can see that the peaks can be scanned by adjusting the ac the phase modulation

parameter. Even when /c = 0, i.e., the phase modulator is not used, the peaks are still movable by adjusting

the frequency modulation parameter /yexcept for that at z = 0. The full width at half maximum (FWHM) of

the peak in terms of distance, which determines the spatial resolution of the system, and the period of the peaks,

which determines the detection range of the system, are given, respectively, as

c c

''reso ~

'y \ 7f ' -range ~ ^ (^)

2.2 Performance deterioration factors and countermeasures

As a technique using discrete modulation, the method suffers the problem of subpeaks in the synthesized

coherence function, which limits the dynamic range of the system. By the phase modulation with properly

designed and /„, the Hanning or Hamming window function can be introduced to suppress the subpeaks. Those window functions can also be realized by the frequency modulation with properly

designed f„ and /„. When modulating the optical frequency directly by the injection current of the LD. the major difficulties come from the nonlinearity between the optical frequency and the injection current amplitude, and the frequency modulation (FM) response of the LD. Both deteriorate the shape of the synthesized coherence

function. In the case of delta-function-like coherence function, the width of the peaks are broaden, i.e., the

sy.stem resolution is deteriorated. The nonlinearity between the injection current and the optical frequency can be compensated by predistortion in the frequency modulation waveform. To do so, an accurate knowledge of the transfer function

between the current and the frequency is needed. We have developed an interferometric method to measure

the dynamic relationship between them. To cope with the FM response of the LD, a countermeasure

employing an inverse filter and an optical gate is developed. The inverse filter compensates the FM response

roughly, while the optical gate suppresses the residual deterioration by masking the transient parts in the modulation steps.

3. HIGH RESOLUTION REFLECTOMTRY BY SYNTHESIS OF

COHERENCE FUNCTION' ^ lo ii i7 i9

A high resolution optical reflectometry is of increasing interest as a powerful tool to evaluate fiber-optic

and waveguide devices. Though various methods, such as the optical low coherence reflectometr>' (OLCR) ^" and the frequency modulated continuous wave (FMCW) reflectometry,"' have been developed for the

92
purpose, there are still some diflQculties, such as the need of a high speed modulation/detection technique,

mechanical moving parts and large amount calculations. Those difficulties will introduce further limitations

in detection ability' and stability.

On the contrary', we have proposed an optical coherence domain reflectometry (OCDR)'' '^^ and a phase

modulated optical coherence domain reflectometry (p-OCDR) ^' ^ by synthesis of the coherence

function. They synthesize the coherence function into the delta-function like peak as discussed in 2.1. When the modulation parameters are chosen so that only one coherence peak exits in the device under test, the

reflected light intensity' at the peak position can be selectively detected. The OCDR does not utilize the phase

modulation, and scans the coherence peak by adjusting the frequency modulation parameters, while the p- OCDR scans the coherence peak by the phase modulation. Generally speaking, the compensation of the

non-linearitv- in the optical frequency modulation in the p-OCDR is much simpler than that in the OCDR.

Besides, the p-OCDR is more tolerant of the non-ideal factors in the optical frequency modulation, because it detects the Oth-order coherence peak of small time delay difference, while the OCDR uses the Ith-order

peak for detection.

Fig. 3 shows the experimental setup of the p-OCDR. It is a heterodyne interferometer with a three

electrode distributed feedback laser diode (DFB-LD) as the light source. The optical frequency is directly

modulated via the injection current into the center electrode. A phase modulation is adopted to scan the coherence peak. The interference signal is detected with the balanced detector and demodulated by the narrow band pass filter (EPF) and the square-law detector. Fig. 4 shows an example of the reflectivity distribution measured by the p-OCDR. The spatial resolution of about 1.2 mm is achieved.

4. LONG R.4NGE REFLECTOMTRY BY SYNTHESIS OF COHERENCE FUNCTION"'

In recent years, for installing and maintaining the optical subscriber networks, measurements of reflectivity distribution at a di.stance of several kilometers with centimeter spatial resolution are required. Among present

methods of reflectometry', it is difficult to realize the cm-resolution for time domain methods. The FMCW

method may be a candidate with its high spatial resolution and wide dynamic range but its measurement

time should get much shorter than its present status for avoiding the optical phase noise due to environmental

fluctuation over a long fiber. On the other hand, we have developed a solution by the synthesis of opfical coherence function as shown in Fig. 5. It is basically the same as the p-OCDR, except for an acousto-optic (AO) switch after the DFB-LD

source. The AO switch generates a series of optical pulse. The optical pulse forms a window to determine a

test region along the tested fiber, because a pulse from the reference path can meet a pulse from the path of the

te.sted fiber only when the two paths have nearly the same lengths. While, the coherence function is synthesized so that there is only one coherence peak inside the region under test, and other periodical

coherence peaks are masked by the window. Therefore, the width of the window is set shorter than the spacing of the coherence peaks. Consequently, the reflecfivity di.stribufion inside the window can be

measured by scanning the coherence peak. For measurement outside the window, the reference delay need

to be changed to determine another region. An example of reflectivity distribution obtained by this system is shown in Fig. 6. Two reflections of around -30 dB from optical connectors can be seen clearly, while the

reflection at the end of the fiber is completely suppressed by the pulse window. The spatial resolution is 6 cm inside a region of 8 m after 5 km optical fiber. The system can test the reflectivitv at one location in one period of the optical frequency modulation

waveform, which is about 80 //.sec in the experiment It corresponds to a frequency sweep-rate of higher than

23 THz'sec in the FMCW method. It is very difficult to realized a continuous linear optical frequency sweep

at that rate with presently available laser devices. We have also demonstrated another configuration using an optical fiber loop with frequency shifter as the

light source, which is shown in Fig. 1'^. The system can select the test region by simply tuning the center frequency of the BPF, instead of changing the delay lines. Figure 8 shows the reflectivity distribution,

measured by the .system with 13cm resolution..

5. CONCLUSION

In this presentation, the principle of the technique "synthesis of opfical coherence function" is summarized.

It has been demonstrated that, by the optical frequency modulafion and the phase modulation, actually

arbitrary coherence function can be .synthesized for difYerent applicafions. One of the most useful synthesized

coherence function is that of the delta-function-like shape, which has found many applications in distributed

93
information detection. In this preseatation. the high resolution and the long range reflectometry have

especially been discussed. Additionally, we have already proposed, with this technique, the 2-D and 3-D

information processing such as optical tomography of scattering medium'^'^'. and distributed and multiplexed

fiber-optic sensing"""^" . In these applications, the technique is featured as a full optical processing with neither

mechanical scanning nor digital processing, compared with the OLCR and FMCW methods. The spatial

resolution of the technique is inversely proportional to the tunable range of the laser source. It is expected

to be improved to several tens of micrometers by using a widely tunable LD, such as the super-structure grating

distnbuted Bragg reflector (SSG-DBR) LD.'^

ACKNOWLEDGMENTS

The authors would like to thank Zuyuan He for his support in the prparation of this manuscript.

REFERENCES

1. D. T. .long and K. Hotate, Proc. OFS-6, pp. 94-100, Paris, 1989.

2. K. Hotate, N. Okumura, M. Higashiguchi and N. Niwa, Opt. Lett., vol. 7, no. 7, pp. 33 1-333, 1982.

3. K. Hotate and D. T. .Tong, AppL Opt., vol. 28, no. 7, pp. 1289-1297, 1989.

4. K. Hotate and O. Kamatani, Electron. Lett., vol. 25, pp. 1530-1505, 1989.

5. K. Hotate and O. Kamatani, J. Lightwave Techno!., vol. 1 1, no. 10, pp. 1707-1709. 1993.

6. K. Hotate. Optical Fiber Technology, vol. 3, no.4, pp.3 56-402, 1997. [Invited]

7. K. Hotate. T. Saida and Z. He, Proc. Intl. Conf. on Applied Optical Metrology), Balatonfiiered, 1997. [Invited]

8. K. Hotate, Z. He and T. Saida, Proc. SPIE's 43rd Annual Meeting, San Diego, 1998. [Invited]

9. M. Bom and E. Wolf, Principle ofOptics, 5th edition. New York: Pergamon Press, 1975, Chapter X.

10. K. Hotate and T. Saida, Proc. OFS-10, pp. 534-537, Glasgow, 1994. U.K. Hotate and T. Saida, Electron. Lett., vol. 3 1, pp. 475-476, 1995.

12. K. Hotate, T. Okugawa and T. Saida, Proc. SPIE, vol. 2507-8, pp. 66-77, Munich. 1995. [invited]

13. K. Hotate and T. Saida. Proc. CLEO Pacific Rim '95, p. 53, Chiba, 1995. [invited]

14. T. Okugawa and K. Hotate, Proc. CLEO Pacific Rim '95, p. 156, 1995.

15. T. Okugawa and K. WoX&Xt, Photon. Technol. Lett., vol. 8, pp. 1710-1712. 1996.

16. K. Hotate and T. Okugawa, J. Lightwave Technol, vol. 12, no. 7, pp. 1247-1255. 1994.

17. O. Kamatani and K. Hotate, J. Lightwave Technol. vol. 1 1, no. 1 1, pp. 1854-1862, 1993.

18. K. Hotate and T. Saida, Proc. SPIE. vol. 2294, pp. 22-31, San Diego, 1994.

19. T. Saida and K. Hotate, Proc. OFS-II, pp. 398-401, Sapporo, 1996.

20. K. Takada, J. L;g;7r>t;ave rec;2«o/., vol. 14, pp. 1677-1689, 1996.

21. U. Glombitza and E. Brinkmeyer, J. Lightwave Technol, vol. 1 1, pp. 1377-1384, 1993.

22. R. Pas.sy. N. Gisin. .1. P. von der Weid and H. H. Gilgen, J. Lightwave Technol, vol. 12, pp. 1622-1630, 1994.

23. T. Saida and K. Hotate, Proc. CLEO '97, pp. 419-420, Baltimore, 1997.

24. T. Saida and K. Hotate, PAotow. Technol Lett., vol. 10, no. 4, pp.419-420. 1997.

25. K. Tsuji. K. Shimizu. T. Horiguchi and Y. Koyamada. £'/ecrro«. Lett., vol. 33. pp. 408-410, 1997.

26. T. Saida and K. Hotate, Proc. lOOC/ECOC '97, pp. 283-286, Edinburgh, 1997.

27. K. Hotate and T. Okugawa, Opt. Lett., vol. 17, no. 21, pp. 1529-1531, 1992.

28. T. Okugawa and K. Hotate, Photon. Technol Lett., vol. 8, pp. 257-259, 1996.

29. T. Okugawa and K. Hotate, Optical Review, vol. 1. no. 1, pp. 8-1 1, 1994.

30. Z. He. T. Okugawa. N. Mukohzaka and K. Hotate, Proc. CLEO '96. CPD4, Anaheim. 1996.

31. Z. He. T. Okugawa. N. Mukohzaka and K. Hotate. Proc. SPIE, vol. 2899. pp. 222-229. Beijing, 1996.

32. Z. He, N. Mukohzaka and K. Hotate, Photon. Technol Lett., vol. 9, no. 4, pp. 514-516. 1997.

33. T. Saida and K. Hotate, P/30to«. Technol Lett., vol. 9, no. 4, pp. 484-486, 1996.

34. B. Zhu and K. Hotate, Proc. CLEO Pacific Rim '97, pp. 257-258, Chiba. 1997.

35. B. Zhu and K. Hotate, Proc. OECC '98. Chiba, 1998.

36. H. Ishii, H. Tanobe, F. Kano, Y. Tohmori, Y. Kondo and Y. Yoshikuni, J. Quantum Electron., vol. 32, no. 3, pp. 433-441, 1996.

94
0 Time delay difterence t

(c) SynthMlzad coharano* (unction

Fig. 1 Basic configuration of synthesis of

optical coherence function, (a) Frequency

modulation waveform, (b) phase modulation

waveform, and (c) synthesized coherence

function.

(a)

f(t). i(t)

Fig. 3 Experimental setup of phasemodulated optical coherence domain

reflectometry by synthesis of the optical

coherence function.

f Af «t>=2itf„K

r NAf I Ujlj-nJlpnr^ I 2n

-time

(a) Optical FM waveform

£ 1

S

«

oO

(b) Optical PM waveform

N=21

0 1 Normalized delay difference t

(c) Synthesized coherence function

ACM

Optical fiber

0 0.05 Distance [m]0.1

Fig. 2 Synthesis and scanning of the

coherence peak by synchronous frequency

modulation and phase modulation.

Fig. 4 Reflectivity distribution measured by

the phase-modulated optical coherence domain

reflectometry.

95
frequency +0 +Af -At+2Af Ml coherence window Pulse window

(width=6cm)

(wi<«h=8m)

HHD

Delay lines

[n]BPF

'^J-' (160MHz)

Output

(a)

Direct FM wavelorm of DFB-LD Phase modulation waveform

for synthesis of the cof>erence peak for scanning tfie cofierence peak

Fig. 5 Experimental setup of long-range

reflectometry by synthesis of the optical

coherence function.

-Ught tourer

0 if 24f »3Ar n n n n .

AOM3

Mod

fluetuallom y

I CXKy Urn

Output

Fig. 7 Experimental setup of long-range

reflectometry by synthesis of the optical

coherence function using the optical delay loop

with a frequency shifter.

5 km

Optical fiber

-20

1 m -[IMI}-

Connector Connector

-, , ,

1

1 1 r

Measuring range (8 m)

6 cm: resolution

20 m a

1 0 1 Distance [m]

>

o

JO. Fiber end

0

0.05 t ' ' ' [ ' ' T ' ' I -^—< -1—1—1—1—1

0.04

003

0.02 13 cm —

0.01 BPF Irequency:

'a r^-4f 0 ... 1 ... 1 ... 1 ... 1 ... 1 .. . 0 20 Distance |m]

Fig. 6 Reflections at two connectors located at 5 km distance measured by the long-range

reflectometry.

Fig. 8 Reflectivity distribution measured at 5

km distance with changing the measurement

ranges.

96
Distributed-Gain Measurements of Erbium-Doped Fibers

J. F. von der Weid, A. O. Dal Forno, J. A. Pereira da Silva and R. Passy

Centerfor Telecommunications Studies - Catholic University ofRio de Janeiro (Brazil)

Rua Marques de Sao Vicente 225 - Gdvea - Rio de Janeiro, RJ 22453-900

M. R. Xavier de Barros

CpqD -Telebrds (Brazil)

B. Huttner and N. Gisin

Group ofApplied Physics - University of Geneva (Switzerland)

Abstract

A non-desctructive optimum gain calibration oferbium-dopedfiber amplifiers was achieved

by using the Coherent Opticalfrequency Domain reflectometry. The Coherent OFDR detection technique

intrinsicallyfilters ASE, allowing precise measurements ofRayleigh Backscattering levels.

Introduction

Optical amplifiers based on erbium-doped fiber have enabled tremendous increases in the

capacity and functionality of optical networks. One of the key parameter in an EDFA fabrication

is the ideal length of the doped fiber, which is normally calculated theoretically [l]-[2].

However, the practical determination of the optimum length for maximum gain is done with a cutback method, which is a destructive technique and very inconvenient due to the high cost of

the Erbium doped fiber. The distributed gain in non-isolated long EDFA's could be measured

with OTDR by employing optical filters to eliminate ASE but only long, of the order of some

kilometers, and low doped fibers could be measured due to the limited resolution of the

technique [3]. Coherent Optical Frequency Domain Reflectometry (C-OFDR) is a very powerful

technique for the characterization of return loss of optical components with high dynamic range

and sensitivity [4]. Because of the coherent detection technique, ASE is naturally filtered so that

C-OFDR is specially indicated for the characterization of optical circuits with gain and ASE

features. Return loss and gain measurements were indeed performed with this technique [5]. Here

we present C-OFDR measurements of the distributed gain of Erbium doped fibers for optimum

length determination for different amplifier applications.

Experiment

C-OFDR is based on the detection of the beat signal generated in a Michelson interferometer

when the optical fi-equency of a laser source is swept linearly. One of the arms gives a reference

97
reflection, whereas the other is connected to the fiber under test. The sensitivity of the technique

is determined by the laser intensity and its intensity noise, whereas the maximum detectable

range is given by the laser linewidth. In our experimental setup we used a three electrodes

semiconductor laser coupled to an external cavity in order to increase its coherence length [6]. When coupled to the external cavity the laser linewidth decreased from ~2 MHz to ~5 kHz, so

that Rayleigh backscattering levels could be measured in the kilometre range. The OFDR probe

power was -14 dBm, and the coherence loss due to the finite linewidth of the laser is r] = 0.013

dB/m, so that the maximum coherence correction within our frill 100 m measurement range will

be 1.3 dB. The spatial resolution depends on the range of the measurement, being ~ 20 cm in the

particular 100 m range presented here. Figure 1 presents the experimental set-up, in which the

detailed configuration of the OFDR reflectometer is the same as described before [6]. The optical

isolator was used when small signal gain measurements were performed. The isolator reduced

the probe intensity to -40 dBm without any further reduction of the Rayleigh backscattering

signal detected by the OFDR. Also 20 m of a standard single mode fibre were spliced to the

WDM pigtail in order to provide a reference Rayleigh backscatter signal. The Fiber under test was a 120 m long, 120 ppm Er^^ doped fibre fabricated at CpqD-Telebras.

Results

Figure 2 shows the small signal gain curves of a 80 m long fiber obtained from the OFDR

measurement of the Rayleigh backscattering signal. Here the probe power was -40 dBm, and the curves clearly shows that the optimum length is strongly dependent of the pump power, the

optimum length being 40 m for 19.8 dBm pump power, and the corresponding gain is 25.7 dB.

The fiber was cut at 40 m in order to measure the gain, which was 24.5 dB, slightly less than the

OFDR result. This difference can be explained by the fact that the cutback measurement includes

splices and connections other than the OFDR, which measures the fiber gain only. Also, the

coherence loss of 40 m contributes with a -0.6 dB additional decrease in the OFDR

measurement, so that the agreement between the two methods is remarkable.

A numerical simulation based on the Er^^ doped fiber parameters was performed in order to

evaluate the gain along the fiber, figure 3. The optimum length obtained by the numerical

analysis shown to be longer than the values found experimentally with average difference around

7 meters. In the worst case, this optimum length difference produced a gain difference of 2 dB,

figure 4. Practically, the use of numerical simulation for the optimum length determination

implies an accurate knowledge of the injected pump power in the Er^^ doped fibre which can be

dificult to determine without a spatially resolved reflectometry technique due to splice losses and

insertion losses on the WMD coupler, filters and isolators.

98
Large signal gain curves are presented in figure 5, and were measured with a modified set-up in which a second laser was connected to the amplifier via an optical 3 dB coupler, launching - 3

dBm into the amplifier. This second laser wavelength was only -50 GHz away from the probe

wavelength (1557.6 nm) so that the depleted gain curve corresponds to the same probe

wavelength. As it can be seen in figure 5, the optimum length in this condition decreased to -20

m with -12 dB gain for the maximum pump power.

Conclusions

The distributed-gain of Erbium-doped fibres was measured with coherent Optical

Frequency-Domain reflectometry. Because of its coherent detection technique, OFDR naturally

filters the ASE within the 100 kHz detection bandwidth. Although filtered, the ASE white beat

noise limits the detector amplifier gain, increasing the noise floor of the measurement. Our

experiments place the OFDR technique as a powerful technique for fibre Lasers and Amplifier

manufacturers for WDM system applications.

Acknowledgements

This work was partially supported by CPqD Telebras under the contract JPqD 779/97, Brazilian

National Research Council CNPq, Swiss CTI Project 3160, and the European ACTS Bliss

AC065 project.

References

[1] F. F. Riihl, "Calculation of Optimum Fiber Lengths for EDFAs at arbitrary pump

wavelengths". Electron. Lett., (1991), Vol. 27, No. 16, pp. 1443-1445.

[2] M. Lin and S. Chi, "The Gain and Optimal Length in the Erbium-Doped Fiber Amplifiers

with 1480 nm Pumping", IEEE Photonics Technol. Lett. (1992) Vol. 4, No. 2, pp. 354-356.

[3] M. Nakazawa, Y. Kimura and K. Suzuki, "Gain-Distribution Measurement along an

Ultralong Erbium Doped Fiber Amplifier using Optical Time Domain Reflectometry", Optics

Lett. (1990), Vol. 15, No. l,pp. 1200-1202.

[4] J. P. von der Weid, R. Passy, G. Mussi and N. Gisin, "On the Characterization of Optical

Fiber Network Components with Optical Frequency Domain Reflectometry", IEEE J. Lightwave Technol (1997), Vol. 15, No 7, pp. 1131-1141.

[5] J. P. von der Weid, R. Passy and N. Gisin, "Coherent Reflectometry of Optical Fiber

Amphfiers" IEEE Photonics Technol. Lett. (1997) Vol. 9, No. 9, pp 1253-1255.

[6] J. P. von der Weid, R. Passy and N. Gisin, "Mid range coherent optical frequency domain

reflectometry with a DFB laser diode coupled to an external cavity", IEEE J. Lightwave

Technology (1995), Vol. 13, 5, 954-960.

99
Length [m]

Figure 2 - Small signal gain curves of a 120 ppm Erbium

doped fiber for co-propagating pump and signal.

Intermediated curves were taken each 3 dB pump level.

Length (m) Pump Power [mW]

Figure 3 - Simulated distributed gain. Figure 4 - Optimum length as fuction of pump power and

gain difference between calculated and measured length.

' 1 ' 1 ' 1 ' 1 ' 1'

1

' 1

15 Pump 19.8 dBm

10

Fiber end

1

'

5 f/ ^^AA/V, Pump 16.4 dBm

0

1 . 1 . I . 1 . 1 0 20 40 60 80 100 120 140

Figure 5 - Large signal gain curves of a 120 m long 120

ppm Erbium doped fiber.

100
POLARISATION OFDR FOR MEASUREMENTS OF

BIREFRINGENCE AND POLARISATION MODE

COUPLING LENGTHS IN OPTICAL FIBRES

B. Huttner, J. Reecht and N. Gisin (1)

R. Passy and J. P. von der Weid (2)

( 1 ) Group of Applied Physics

University of Geneva

20 Rue de L'EcdIe de Medecine,

CH 1211. Geneva 4, Switzerland

(2) Center for Telecommunication Studies

Pontificia Universidade Catolica do Rio de Janeiro Rua Marques de Sao Vincente 225, Rio de Janeiro, 22453-900 Brazil

Abstract: Measurements of the intrinsic birefringence of optical fibres is performed at 1550 nm using

the Optical Frequency Domain Reflectometry technique. Polarisation mode coupling length is obtained from

local birefringence and polarisation mode dispersion measurements.

Introduction

Polarisation Mode Dispersion (PMD) is a statistical quantity describing the effects of the distributed birefringence in the distortion of a light signal along an optical fibre. In the theoretical modeling of this phenomenon the fibre can be described as a concatenation of pieces of homogeneous fibres with a mean modal

birefringence B {B is generally expressed in ps/km) [1]. Their mean length is known as the coupling length h. For fibres which are long compared to /?, the PMD of the fibre is equal to Byfhl . so that the knowledge of this parameter h is very important in the transmission properties of single mode fibres. Optical Frequency Domain Reflectometry is well known as a tool for optical fibres and devices

characterization [2], In this technique, the frequency of the laser source is linearly swept as a function of time,

and the reflected field under investigation interferes coherently with a fixed reflection, known as the local oscillator (LO) field. Due to the linear frequency sweep of the source, for each reflection in the fibre there is a corresponding beat frequency, so that the intensity of the reflected field from each point can thus be obtained as the Fourier transform of the interference signal. Since the state of polarisation (SOP) of the reflected field with

respect to the LO affects the intensity of the interference signal, OFDR is naturally a polarisation sensitive reflectometer. In this sense, the OFDR is a useful complement to the Polarisation OTDR (POTDR) [3], and

enables to measure short-scale effects, albeit at a much shorter distance than the POTDR [4]. In this work, we

use the OFDR technique to analyze polarisation effects in the Rayleigh backscattering signal in order to get information on the local birefringence along the fibre. For one homogeneous trunk of fbre with modal birefringence fi, the theoretical model [5,6] predicts that the Rayleigh backscanering signal detected by the OFDR oscillates with two characteristic periods. Lf, and

/./,/2. where L/, = }./cB is the polarisation mode beat length'. Intuitively, L^/l appears because the signal

travels forth and back, so that the effective length of fibre is double. appears when the polarisation of the LO

or the polarisation at the input is parallel to the birefringence axis. The relative intensity of these two components therefore depends on the polarisation. For a real fibre, modeled as a concatenation of homogeneous

trunks, this intensity changes during the propagation, due to the random mode couplings. The determination of

' Note that this is valid only when the birefringence An is independent of the frequency, which is a very good approximation

tor optical fibres.

101
these two periods enables to obtain /,/, , and through it the local birefringence. The combination of this measurenient and of the PMD enables to determine the polarisation mode coupling length of the fibre.

Experimental procedure

The description of our OFDR has been given in previous works [2], A standard single mode fibre, with a rather high value of the PMD, was used to demonstrate the polarisation mode coupling length determination. The

PMD was measured by the interferometric method [1], to give 1.9ps/Vkm . A 5-m long piece of this fibre was

kept loose on the optical table, to avoid any bending effect. The far end refiection of the measured fibre was

eliminated in order to reduce the noise fioor due to the phase noise of the laser. The fading noise of the Rayleigh

backscattering intensity was eliminated by averaging the FFT spectra while slowly sweeping the laser central wavelength [2]. The full wavelength range, (< 1 nm) was small enough to average out the random fluctuations of the Rayleigh backscattering intensity without disturbing the polarisation state of the reflected field.

Results and discussion

The result of the OFDR scan is presented in Fig. 1 where the oscillations induced by the local birefringence can be clearly seen. A Fourier Transform of this curve is shown in the inset, and gives the frequency of these

oscillations, about 3.3 m'' . A small second peak, at 6.6 m'' , is an indication that the main peak corresponds to the beat length Lj, , while the second peak corresponds to Lf, / 2 . The fact that both peaks appear in the Fig. 1 helps identifying the correct beat length to be L,, = 30 cm . This would correspond to a PMD of 17ps for a 1 kmlong optical fibre with no mode coupling, a value much higher than the one measured experimentally. This

confirms that this fibre has some mode coupling. The equation linking the PMD, the intrinsic birefringence and

the coupling length is derived in [1]. For the case of a coupling length h which is small with respect to the fibre length / we get: PMD = —V^. (1)

From Eq. (1), we derive the value of the coupling length: 12 m . We emphasize that a measurement of the PMD alone cannot give the value of this parameter. For example, a low value of the PMD may be due either to a low intrinsic birefringence, or to a small coupling length. Our measurement thus enables a more complete

characterization of the optical fibre. The same setup can be used for the determination of induced birefringence. For example, we have also performed experiments with optical fibres wrapped around spools of various sizes. The typical variation of L/,

as R' , where R is the radius of the spools was observed [7]. It may be also possible to use the OFDR to measure the twist-induced birefringence, as was done in [8].

Conclusion

We have used the OFDR to analyze polarisation effects in optical fibres. Measurement of the intrinsic birefringence, together with a measurement of the global PMD of the fibre enabled us to estimate the coupling

length of the fibre. This work is an example of the use of the OFDR as a complement to the POTDR. which

gives information on polarisation effects in optical fibres on a much larger scale (resolution of several meters, range of several kilometers). The OFDR is well adapted to measurements with range below one hundred meters, and resolution of a few centimeters.

Acknowledgements

This work was supported by Swiss CTI Project 3 1 60.

and by CpqD Telebras under the contract JPqD 779/97

241 partners.

References

[1] N. Gisin, J. P. Von der Weid and J. P. Pellaux, "Polarisation mode dispersion of short and long single-mode

fibres", J. Lightwave Technol., 1991, vol. 9, pp. 821-827.

[2] J. P. von der Weid, R. Passy, G. Mussi and N. Gisin, "On the characterization of optical fibre network

components with optical frequency domain refiectometer", J. Lightwave Technol. 1997, vol. 15, pp. 1131-1141.

, by the European ACTS under the BLISS AC065 project,

. We acknowledge stimulating discussion with our COST

102
[3] A.J. Rogers, "Polarisation-optical time domain reflectometry: a technique for the measurement of field distributions", Appl. Opt. 20, pp. 1 060- 1 074 ( 1 98 1 ).

[4] A. Tardy, M. Jurczyszyn, F. Bruyere, M. Hertz and J.L. Lang, "Fibre PMD analysis for optical-fibre cable using polarisation OTDR", in Optical Fibre Communication Conference, 1995 OSA Technical Digest Series (Optical Society of America, Washington DC, 1995) pp. 236-239.

[5] E. Brinkmeyer, "Forward-backward transmission in birefringent single-mode fibres: interpretation of

polarisation-sensitive measurements". Opt. Letts 6, pp. 575-577 (1981).

[6] E. Chausse, N. Gisin, Ch. Zimmer and B. Perny, "Polarisation mode dispersion analyses with polarisation

OTDRs", in European Fibre Optic Communications and Networks 1995 (The European Institute for Communications and Networks, 1995) pp. 185- 189 (Note that in Eq. (2) in this work, a term: cos(a)fir)(p e_, ) is missing in the squared expression).

[7] R. Ulrich, S.C. Rashleigh and W. Eickhoff, "Bending-induced birefringence in single-mode fibres", Opt.

Letts. 5, pp. 273-275 (1980).

[8] R.E. Schuh, J.G. Ellison, A.S. Siddiqui and D.H.O. Bebbington, "Polarisation OTDR measurements and

theoretical analysis on fibres with twist and their implications for estimation of PMD", Elec. Letts. 32, pp. 387-

388 (1996).

Figures

Length [m]

Figure 1 POFDR measurement of a single mode fibre. The oscillations in the Rayleigh backscattering are due to the

intrinsic birefringence of the fibre. The main peak in the Fourier transform corresponds to a periodicity of 3.3 m'' , whereas the double frequency is barely visible at 6.6 m"'. The polarisation beat length is L^, = 30cm , while

the second peak corresponds to L/, / 2 .

103
, i r

104
BEAT-LENGTH MEASUREMENT IN HIGHLY MODE-COUPLED

SINGLE-MODE FIBERS BY MEANS OF FOURIER TRANSFORM

OF BACKSCATTERED SIGNAL

Fabrizio Corsi, Andrea Galtarossa, Luca Palmieri

DEI Universita di Padova, Via Gradenigo 6/A,

35131 Padova ITALY

Abstract We present a new method able to determine the mean value of beat length in highly

polarization-coupled, randomly perturbed, single-mode fibers. Our method is based on the analysis of the power spectrum of the backscattered signal. Numerical results fit very well with

theoretical predictions

.

Introduction Nowadays, backscattering technique is the most diffused method for optical fiber

characterization regarding losses in optical link up to one hundred of kilometers in length. Its

success is due to its simplicity, high dynamic range and, mainly, because it is a one-end measure. Moreover, it has been also shown that polarization sensitive backscattering technique [1][2] could

be a suitable tool to perform beat length measurement for fiber in so called short-length regime,

i.e. when the correlation length Lq is large compared to the fiber length Z {L/Lr-^Q) [3][4][5].

Recently, we presented an original theory to show the capability of backscattering technique to perform complete polarization mode dispersion characterization of fiber also in long-length regime

{LJL(r^<Xi) considering spatial evolution of Stokes parameters of the backscattered signal as a

fimction of scattering point through the fiber [6]. We have shown also that our method permits to measure die mean beat length Lb and the polarization mode coupling using a polarization sensitive

backscattering technique, simply considering the ratio between the number of extrema (Ne)

compared to the number of fixed-level crossing points (iV^,) [7], v being a fixed value of the

normalized backscattered signal (0< v < 1). We used a linearly polarized input field, and we placed

a linear polarizer in front of the photodiode in detection; polarizer axis was perfectly aligned with

respect the state of polarization on the input signal. We found that for fiber in low-polarization

coupling regime it results Lb=4 L/Ne, while for fiber in high-polarization coupling regime it results Lb=4Lv'-/N..

The aim of this work is to present a new method based on analysis of the Fourier transform of the

backscattered signal that permits to calculate the mean value of Lb. In particular we find that the mean value of the beat length is strictly related to inverse of the standard deviation (STD) of the

power spectrum.

Theoretical background The 3 components of the normalized Stokes vector 5(z), representing

the evolution of the state of polarization of a monochromatic field propagating through a loss-less,

randomly perturbed, single-mode fiber can be related to the input one So , by means of

S{z) =RSo , z being the position along the fiber, while R=R(z) is the MuUer matrix of the fiber.

105
In a similar way, we may introduce S b{::) that is the Stokes vector of the backscattered field. If we use a quasi-monochromatic pulse, the backscattered signal may be written as follows [7]:

5/j(r) = MR''MR6^„ (1)

where, ^ is the transpose and M=diag( 1 , 1 ,-1) a diagonal matrix. From now on, subscripts F and B

refer, respectively, to forward and forward-backward propagation.

Let us place a polarizer having Stokes vector p in front of the photodiode. The normalized power

of the signal passed through the polarizer is given by [7]: f{z)=^[l + Ss{z)-p]. (2)

Because of randomness, R and, consequently, T{z) are stochastic process. We show hereafter

that, under suitable hypothesis, it is possible to retrieve information on the mean beat length

considering the standard deviation (STD) of Fourier transform of the backscattered signal.

It can be shown that R(r) can be considered asymptotically stationary as z increases so that the

probability density function of T at a distance z converges to a r-independent function [8]. Because statistical description of T{z) depends on the mutual orientation of polarizer with respect

to state of polarization of input signal, we will assume the input field to be linear polarized and the

output polarizer to be perfectly aligned to the input state of polarization (p ~ So); consequently, it

results < r(z) >= 2 / 3 [7]. Let us consider a mean-shifted transmission T{z) = T(z) - 2/3 , that

has a zero-mean value ( — 2 / 3 < T{z) < 1 / 3 ). The transmission T(z) will be treated as an ergodic

random process following the same procedures used in [9] to compare the differential group delay

measured by means of interferometric method with respect to fixed polarizer method.

Our results will be valid for a fiber in long-length regime that means the length A z of the fiber

under test is much larger than any variations in T(z) . Let us introduce

Hiz)= jn^)Ti^ + z)d^^AzR^{z ) (3)

At where i?^(z)is the autocorrelation of T(z) that coincides with the expected value of

T{r\)T(r\ + z) . Moreover, we define /z(co.)the Fourier transform of H{z) ; since /z(co.) is an even function its variance cj] reads - - J -

(4)

'CO. 471- H{0)

where H"{0) is the second derivative of H(z) evaluated at z = 0 . Please note that with respect to

[9], there is a factor 1/4;:' because of a different definition of the Fourier transform.

In [8] we found that the state of polarization of forward signal may be completely defined by

Sf - {cos^,cos\\j,cost^in\\),sin^,) , where £, and y are random variables, [-Tr/2,7i/2] and

V|/ e [0,2Tr], whose probability density functions (PDFs') read, respectively,

/,(y)~ = (5) z Zn

Moreover, T and T' may be written as follows:

106
A r = ^(i + co520 r = -(p, •i05/«2^, (6)

where u = (sin\\f -cos\]J ,0) , ^={^^,^2y^}) is the local birefringence vector while = (P, ,0) is the vector that describes the linear component of the local birefringence.

In highly mode-coupling regime, the correlation length of the perturbation affecting the fiber is much smaller with respect to the beat length. Consequeiitly, the state of polarization of the signal

may be considered statistically independent of the value assumed by Pi(-), whenever

z>z/. Furthermore, the two components Pt(z), i.e. P, and p., are assumed to be Gaussian

variables, statistically independent, with zero mean value, and with the same standard deviation ap. By means of relationships introduced in [9] H{0)^Az{r-) H\0) = -/Sz{T") , (7)

we may obtain the following results:

//(O) = ^z{T-) = E[cos' ^] = Az^ (8)

and, consequently, In'al = Sa^ . Let us define the mean value of the beat length. Lb, as follows:

Lg =2tiI E[\ Pi |] . Because | P^ | depends on two statistically independent Gaussian variables, it

is Rayleigh distributed. As a consequence, the mean value of the beat length reads:

1.96

(9)

Numerical results We tested numerically eq. (9), modeling the fiber by means of a cascade of

randomly rotated waveplates [7] so to obtain a fiber of total length of 26,624 m. Additional

parameters introduced were the sampling length Ls that defines the spatial resolution, i.e. the

distance between two adjacent scattering points, and the rotation length Lr i.e. the length of each

waveplate. For all the examples reported in the following we tested numerically that Sf{z)

covered uniformly the Poincare sphere, to be sure that fibers were in long-length regime.

Numerical results were averaged over an ensemble of 100 fibers and are summarized in TAB.

I

{Lb>Lr) and TAB. 11 {Lr>Lb). TAB.I and TAB.II show that expected values of beat length are very

close to theoretical predictions when Ls«min{LB.L/i} . It means that the spatial resolution has to

be small enough compared to longitudinal fluctuations of the fiber. A reasonable accuracy may be

TAB. I

Lsim) 13.0 6.5 3.25 1.625 0.8125

Laim) 103.7 49.4 22.1 15.1 14.9 Leim) 100.6 44.4 30.0 29.5 29.7

Lg (m) 94.1 49.2 43.8 44.3 44.6

(Lii=I3m, L=26,624m; T.V. is the theoretical value ofLb)

107
TAB. II

Lsim) 13.0 6.5 3.25 1.625 0.8125

Lb (m) 101.8 50.5 24.4 11.0 8.1

Lb (m) 101.6 48.7 22.1 16.1 16.0

Lb (m) 98.0 44.2 32.9 31.8 31.8

T.V.=8.0

T.V.=16.0

T.V.=32.0

{LR=52m, L-26,624m; T.V. is the theoretical value ofLb)

achieved when Ls<min{LB,LR]l5. Furthermore, even if our theory applies rigorously only

to highly polarization-coupled fibers {Lr<<Lb), numerical results seem to be close to

theoretical ones for fibers in long-length regime, independently of the ratio Lr/Lq.

Conclusion We presented a new technique able to measure the mean value of the beat length of a

single-mode randomly birefringent optical fiber by means of the power spectrum analysis of the

backscattered signal. We used a linearly polarized input signal in transmission and a linear

polarizer followed by a photodiode in detection. Polarization axis of the linear polarizer was

perfectly aligned with the state of polarization of the input signal. We showed that the mean value

of the beat length is strictly related to the standard deviation of the Fourier transform of the

backscattered signal passed through the polarizer. In particular, the larger the STD of the

backscattered signal, the smaller the mean value of the beat length. Numerical results fit very well

theoretical predictions when spatial resolution of backscattered signal is small compared to spatial

scale of fluctuations of fiber under test.

Acknowledgements We thank prof C. G. Someda for fruitful suggestions. Our work was partially

supported by an EEC/ ACTS/ ESTHER project. It is also part of cooperation agreement between

CSELT and Consorzio Padova Ricerche.

References

1. Rogers A. Applied Optics, pp. 1060-1074, 1981.

2. Kim B. Y. and Choi S.S., Optics Letters, pp. 578-580, 1981.

3. Galtarossa A., Pistolato P., Schiano M., Proc. EFOC&N. pp. 190-192, 1995.

4. Ellison J. G. and Siddiqui A. S., IEEE Ph. Tech. Letters, pp.246-248, 1998.

5. Poole C. D., Nagel J., in Optical Fiber Telecommunications III A, I. P. Kaminow and T. L. Koch eds. Academic Press, 1997

6. Galtarossa A., Corsi F., Palmieri L., Proc. OFC'98, San Jose (CA), paper ThR4.

7. Galtarossa A., Corsi F., Palmieri L. , "Polarization mode dispersion characterization of singlemode optical fiber using backscattering technique", accepted for publication in IEEE J. Light.

Technology.

8. Corsi F., Galtarossa A., Palmieri L., "Beat-length characterization based on backscattering

analysis in highly polarization coupled single-mode fibers ", to be published.

9. Heffner B. L., Optics Letters, pp. 1 13-115, 1996.

108
Spun fibre parameter extraction using Polarimetric Optical Time Domain

Reflectometry

J.G. Ellison and A.S. Siddiqui

Department of Electronic Systems Engineering, University of Essex, Colchester C04 3SQ, UK

Abstract

The spatial frequencies present in a polarimetric optical time domain reflectometer trace of spun fibre are a strong function of external twist and initial spinning parameters. In this paper we show how

analysis of the spatial frequencies in the POTDR trace in conjunction with the backscattered state of

polarisation evolution can determine the spinning parameters of the spun fibre.

Introduction

The recent commercial availability of spun fibres has enabled a significant reduction in the

Polarisation Mode Dispersion (PMD) of newly deployed systems. These spun fibres are produced by

applying a sinusoidal torque to the fibre being drawn, alternatively in the clockwise and anti- clockwise direction. This has the effect of 'scrambling' the linear birefringence axes and hence

reducing the effective linear birefringence. In [1] we presented a technique for estimating the residual linear birefringence within the spun fibre using Polarimetric Optical Time Domain

Reflectometry (POTDR) [2], and hence provide an indication of how effective the chosen spinning

parameters were for a particular fibre sample. In this paper we show how examining the twist

dependency of the spatial frequencies and the state of polarisation (SOP) evolution in the POTDR

trace allows information about the spinning parameters themselves to be extracted. This singleended, non destructive technique may be of use to fibre manufacturers as a check to ensure that the

spinning process is imparting the correct spin to the fibre itself.

Theory

The backscattered SOP evolution has been computed by modelling the fibre as a series of linear

retarders whose angle of azimuth varies sinusoidally along the fibre in accordance with the spin

period, A.^, and the spin amplitude, A^. Each of the linear retarders has a linear birefringence

determined by an initial, unspun linear birefrigence, Pl- The application of external twist, y, has been

included in the model by an additional azimuth rotation of the linear retarders following the twist,

and a circular birefringence component related to the external twist by the stress-optic coefficient, g. The spatial frequencies present in the backscattered SOP evolution were obtained by performing a

Fast Fourier Transform upon each of the Stokes parameters.

Fig. 1 shows the backscattered spatial frequencies as a function of externally applied twist for an

initial linear birefringence, Pl, of 2.5 rad/m, a spin period, A.^, of 2 metres, and a spin amplitude, A,^, of 1 turn. The occurrence of a spatial frequency at 0.5 cycles/m for most values of twist is immediately obvious and corresponds to the spin period, A^. Extraction of the other two parameters,

the spin amplitude and mitial linear birefringence, is more difficult however. A large number of

simulations were performed with various starting parameters to attempt to uncover a systematic trend. The height and position of the 'eye' patterns varies strongly with A.^ and A.^, and less so with Pl, but

other than the regular appearance of a constant spatial frequency at 1/A.^, a systematic pattern is

difficult to see. Extraction of A.^ and Pl from the spatial frequency map of a real fibre must therefore

be performed by best fit matching against the simulated spatial frequency map whose parameters are chosen by trial and error. This process may be hastened by a comparison between the measured and

simulated SOP evolutions at key twist rates, namely at the maximum opening of an 'eye' and at the crossover points between 'eyes'. The size of the characteristic looped figure of eight pattern at the

109
'eye' crossover points is determined by the magnitude of and (3l and helps provide a starting value

and constraint for the values used in the simulation.

Results

A spun dispersion shifted fibre was suspended in a 30m long catenary such that a known twist could

be applied to the fibre. The POTDR [2] was used to measure the backscattered SOP evolution and

spatial frequencies for a number of different twist rates. From the spatial frequency data, the spin

period, A.^, was identified as 1.65m, and an examination and comparison of the SOP evolution against

trial simulation data provided a range of possible values for the spin amplitude and initial linear

birefringence. The best match from a range of simulation runs was fine-tuned until the simulated

spatial frequencies and SOP evolution closely matched the measured spatial frequency data and SOP

evolution. Fig. 2 shows the final spatial frequency map for a linear birefringence, Pl. of 3 rad/m and

spinning parameters A^ = 1.1 turns and A^= 1.7m. The simulated and measured SOP evolution for a

particular twist rate is given in figs. 3a and 3b and shows a reasonable agreement in the overall shape.

Note that the SOP evolution becomes very convoluted and shows a strong sensitivity to the spinning

parameters themselves. It is therefore unreasonable to expect an exact agreement between the SOP

evolutions in theory and practice due to small irregularities in the spinning process. As a check of the accuracy of the extracted spinning parameters, the differential group delay (DGD)

versus twist characteristics were measured using the Jones Matrix Eigenanalysis technique and

compared with the theoretical plot obtained by a numerical method [3] for the given spinning

parameters (figs. 4a and 4b). As can be seen from the results, the measured and theoretical DGD

plots match well, giving some confidence to the values of the spinning parameters extracted. A sample of the same fibre produced without spin was also measured with the POTDR to confirm a value for the initial linear birefringence, |3l. This was found to be 1 .7 rad/m, a value different from

that obtained in the spun fibre. The discrepancy may be due to the mechanics of the spinning process

itself which may increase the internal linear birefringence.

Conclusions

We present a technique for extracting the spinning parameters used in the manufacture of a spun fibre

using POTDR. Although the technique is not perfect yet, with some refinement it should be of use to fibre manufacturers to help verify that the spinning process is imparting the correct spin to the fibre produced.

Acknowledgements

This work was carried out under EPSRC Research Grant number GR/K49454. The industrial

partners are BT Laboratories, Martlesham Heath, Ipswich, UK and Alcatel Submarine Networks,

Greenwich, London, UK.

References

[1] J. G. Ellison and A. S. Siddiqui, "Estimation of linear birefringence suppression in spun fibre using polarimetric optical time domain reflectometry," Conference on Lasers and Electro-Optics

(CLEO '98). San Francisco, paper no. C Th 057, 1998.

[2] J. G. Ellison and A. S. Siddiqui, "A Fully Polarimetric Optical Time Domain Reflectometer,"

IEEE Photonics Technology Letters, vol. 10, no. 2, pp. 246-248, 1998.

[3] R. E. Schuh, X. Shan and A. S. Siddiqui, "Polarisation mode dispersion in spun fibres with

different linear birefringence and spinning parameters," 4th Optical Fibre Measurement Conference

(OFMC '97), NPL, Teddington, London, pp. 122-125, 1997.

110
0.7 —

0.6-

f

<u 0.5 —

u

u

Twist (turns/m)

Fig. 1 Simulated spatial frequencies versus twist for Pl = 2.5 rad/m, A.^ = 2m and A., = 1 turn. Density

of print represents the normalised amplitude at that frequency.

0.7

Twist (turns/m)

Fig. 2 Simulated and measured (O) spatial frequencies versus twist for Pl = 3 rad/m, A.^= 1.7m and

A., = 1.1 turns.

Ill
(a) LCP (b) LCP

RCP RCP

Fig. 3 Backscattered SOP evolution at a twist rate of 1.75 tums/m, (a) measured and (b) simulated

with Pl = 3 rad/m, = 1 .7m and = 1.1 turns.

Fig. 4 DGD versus twist characteristics, (a) measured and (b) simulated with Pl = 3 rad/m, A.^ = 1.7m

and A., = 1.1 turns.

112
Characterizing Multimode Fiber Bandwidth for

Gigabit Ethernet Applications

(Invited)

Michael J. Hackert

Coming, Inc., SP-BN

Corning, NY 14831

Introduction

The demand for ever increasing volumes of data transmission is driving new development in the data communication marketplace. Sources, fiber, receivers, and connectors all are being re-engineered to provide low cost solutions to fill the needs of the market place. This paper describes the latest, on going

technical developments regarding the application of multimode optical fiber for data communication.

The private premise or fiber to the desk (FTTD) market has two underlying needs: one is for low costs and

the other is for industry accepted standards. New technologies such as vertical cavity surface emitting

lasers (VCSELs) provide the promise of low costs (comparable to light emitting diodes - LEDs) combined

with superior performance previously unobserved'. For example, VCSELs can easily operate at 2 Gigabit

and produce milliwatts of power with threshold currents so low that they can be driven directly from logic

gates without the need for drivers. These sources operate at short wavelengths around 850 nm. Standard

single-mode fiber (SMF) is not designed to operate at this wavelength whereas multimode fiber (MMF) is ideally suited. In addition, low cost silicon detectors provide good responsivity at this wavelength as well

as high speed. An additional benefit is that the VCSEL manufacturing process provides the promise of

readily controlling their launch power distributions. The Gigabit Ethernet standard, which recently was

approved under the direction of the IEEE 802. 3z committee, is providing the standardization the marketplace wants. Thus, the stage is set for renewed interest in multimode fiber. The combination of low

cost connectors, inexpensive sources and detectors, and multimode fiber provides a cost-effective solution

for current data communication needs.

In order to apply multimode fiber to higher speed applications, numerous technical considerations, which

had not been significant during the development of slower speed system standards such as Fiber Distributed Data Interface (FDDI)", required investigation and resolution. The Telecommunication Industry

Association (TIA) responded to industry's need by providing a forum for various participants to come

together to do the fiber and source development (TIA FO-2.2 Task Group on Modal Dependence of

Bandwidth). The IEEE provided the forum to develop a gigabit system standard (IEEE 802. 3z). The

results reported herein come from work of each of those groups. DMD and Bandwidth Fundamentals

Multimode fiber's information carrying capacity typically is rated in terms of a bandwidth length product

(MHz-km) that can be used to determine how far a system can operate at what bit rate^. A simple intuitive model for the bandwidth characteristics considers the fiber to consist of a number of discrete delay lines, each of which corresponds to a particular mode'*^ ''. A conceptual model is shown in Figure 1. In the

figure, the low order modes (LOM)

correspond to the modes or rays propagating

down the center of the fiber; the high order modes (HOM) propagate near the core/clad

interface; and the intermediate modes

propagate in between. Although somewhat

counter intuitive, the intermediate modes

carry most of the power. Figure l- conceptual Model for Multimode Fiber Bandwidth

In an ideal fiber, all of the delays are tuned to be identical. Thus, when a temporally narrow pulse of light

is launched into the fiber, its shape is maintained at the output. However, as the example in Figure 1 shows, the over exaggerated delay error causes the output pulse to be broadened. The high order mode

113
power arrives late relative to most of the power (in the intermediate modes), and the low order modes arrive

early. Thus, the bandwidth is reduced and the information carrying capacity is limited.

This model then explains why bandwidth is launch dependent. The output pulse given in Figure 1 corresponds to an overfilled launch where all of the modes are excited with the maximum amount of power

they can carry. This launch is defined by standards^; and a typical source that this might correspond to is an LED. If the launch power distribution is then reduced so that only the lower and intermediate modes are excited, the power in the late peak of the pulse disappears, the pulse width decreases, and the bandwidth

goes up. The analogy can be extended equally as well where the launch is restricted to just the lowest order modes and the output pulse becomes very narrow. Thus, if all of the modes of a multimode fiber are not tuned perfectly, the bandwidth can change as the launch power distribution changes.

The tuning of these modes is quantified by the differential mode delay (DMD) measurement. The DMD is a process tuning tool which measures the delay of each of the modes. It has never been standardized

because it is difficult to implement, costly to run, and not a functional input measure for system modeling.

Bandwidth, on the other hand, is functional and is used in system modeling'. The DMD consists of

recording the mean delay as a spot of light formed by a single-mode fiber is scanned across the input of the

fiber. The typical technique for this measurement is is a variation of the time domain bandwidth

measurement^ - a high speed pulse is used. The US

National Institute of Standards and Technology

(NIST) recently reported on improved resolution using a frequency domain technique that allows

measurement of short lengths (approx. ten meters or ±0.3 ps/m)^. An example is given in Figure 2 where

delay (given in psec/m or nsec/km) is plotted versus

offset of the single-mode spot where positive delay

corresponds to increased time.

OflMt (micront)

Figure 2 - Differential Mode Delay tor TIA TG 2.2 Round Robin

Cable (Fiber 7)

The DMD measurement effectively is a very precise measure of the graded index of a multimode fiber. Measures of the index such as the Refracted Near- field (RNF) measurement^ fail to have sufficient resolution to see the delay errors that are significant for tuning a multimode fiber. Figure 3 shows an example of a perfect alpha profile and a perturbed profile which reduced the modeled bandwidth by

approximately a factor of two. It is evident from the figure that the difference is barely perceptible.

Typical RNF measurement equipment has

insufficient resolution to measure such a small difference (just another challenge to tuning for increased bandwidth).

Fortunately, a theoretical foundation to model bandwidth from DMD data was

completed during early fiber development"*^^. The models provide

sufficient accuracy to estimate average performance when empirical adjustments

are made. However, the precision is somewhat limited.

Radial Position (microns) Rgure 3 - Theoretical Bandwidtfi Impact to Index Perturbation

800 SCO KXS 1100 1200 1300 1«0 1S0O Wavelength (nanometefs)

Figure 4 -Theorelical BancMndth for (>aded Irviex cere MJtjrrcxJe Fbers

The maximum theoretical bandwidth of a given multimode fiber design is limited by

the maximum number of modes. There

are approximately 2.5 times as many

modes in 62.5 |im fiber as there are in 50

|j.m fiber. This can be seen in Figure 4

where the maximum theoretical bandwidth

is plotted versus wavelength.

Consequently, higher bandwidths can be

achieved with 50 |im fiber. This is traded

114
off against the greater light gathering abihty of the larger core (62.5 fim versus 50 ^im) and larger

numerical aperture (NA) (0.275 versus 0.20, respectively). This makes 62.5 ^m fiber ideally suited for LED based applications since LEDs typically have large NAs and spot sizes, and 50 |im fiber ideally suited

for laser based applications.

The index of the glass varies as a function of wavelength. This gives rise to the relationship between

bandwidth and wavelength shown in Figure 4. The fiber designer can move the wavelength of peak

bandwidth through the tuning process. Key applications in the last ten years have focused attention to 1300 nm operation with a specification of 160 MHz-km at 850 nm and 500 MHz-km at 1300 nm on 62.5 \im

fiber (FDDI grade). This has moved the wavelength of peak bandwidth for 62.5 }J.m fiber to 1300 nm. 50

|im fiber has sufficiently high bandwidth that by tuning the fiber for peak bandwidth approximately

halfway between 850 and 1300 nm, 500 MHz-km can be achieved in both windows.

At the wavelength of peak bandwidth, the DMD profile for an ideal fiber is flat. However as we move off

this peak wavelength, a systematic error in the DMD profile becomes evident. The DMD profile shifts upward or to later delays for shorter wavelengths and downward or earlier delays for longer wavelengths.

The intuitive model given in Figure 1 corresponds to the latter, longer wavelength case. This wavelength

relationship suggests that for restricted launches produced by lasers, the 850 nm bandwidth should increase

relative to the overfilled launch for 62.5 |im fiber; and the bandwidth at both wavelengths for 50 |im fiber should increase. Exploitation of this opportunity has been a topic of standards development for the last couple of years and its investigation is ongoing.

Development of a New Multimode Standard

Besides the development of new, short wavelength source technology, the development of the Gigabit

Ethernet Standard beginning around 1995 was another driving force. Its need was simple - go faster, farther, and cheaper than past standards to fill the current channel capacity constraints. Miscellaneous

laboratory experiments showed that significantly higher bandwidths could be achieved when the reduced

launches of the new system sources were used with standard multimode fiber. In January of 1996,

interested companies formed the TIA FO-2.2 Task Group on Modal Dependence of Bandwidth. Its objective was to determine if a means of ensuring consistent improved performance could be developed

through a combination of both control of the source to produce a consistent restricted launch condition and

an additional fiber characterization which confirms a new minimum performance with this controlled

launch.

Some of the initial results (circa 6/96) are given in Figure 5 and Table 1. A histogram is graphed for the increase in fiber bandwidth at 850 and 1300 nm (expressed as a ratio) between a highly restricted launch

(namely that produced by SMF) and an overfilled launch. Data for 26 fibers are graphed and the mean and

range are tabulated. The average bandwidth is approximately doubled. This held out the promise that a

solution could be found.

Table 1 850 nm 1300 nm

Mean 2 Ox 2.8x Range 1.1 to 10.8x 1.1 to 5.0x Count 26 26

Note: 1300 nm values were limited by the maximum

measurement ability of the measurement system.

iriri

8S<)nm

XL

<l llo 2 2 10 3 3 10 4 4io5 5lo6 6lo7 7lo8 >8

Gam Factor Figure 5 - Bandwidth Increase for 62.5 um MMF using an SMF

Launch

The first key action of the Task Group was to implement an inter-laboratory measurement round robin. Its objective was to evaluate the inconsistencies in observations between laboratories by providing a consistent sample set to participants and a well defined set of launch conditions. A secondary objective of the round

robin was to gain additional characterization information of fiber performance with actual system sources. Towards this end, a round robin cable was fabricated consisting of nine 62.5 |im fibers and six 50 |im

fibers. The fibers were selected to incorporate the widest range of performance while attempting to select standard grade fiber. The bandwidth criteria attempted to include fibers with a wavelength of peak

115
bandwidth at 850 nm, 1300 nm, and in between 850 and 1300 nm. A 3 km length was fabricated and 300

meter sister pieces were shared with participants. All fibers were assumed to be identical. A number of launch conditions were included in the round robin. They included:

1 ) the standard overfilled launch (OFL per FOTP 54^),

2) a number of conditions thought to reflect actual launch distributions by lasers (not to mention

expected to be easily implemented and anticipated to be reasonably robust),

a) a 50 |im spot created by overfilling a 50 mm fiber and

b) a 30 |im spot generated by applying a five turn 5 mm diameter mandrel to an overfilling 50

|im fiber, and

3) actual direct connection measurements using transceivers

a) VCSEL and compact disk (CD) lasers in the 850 nm window and

b) Fabry Perot (FP) lasers designed to operate with SMF (and recommended for the Gigabit

Ethernet standard for both fiber types).

Eight participants performed testing, and NIST supervised the round robin and analyzed the large amount

of information which was collected.

The results of this analysis are given in Figures 6, 7, and 8. Figure 6 and 7 show the percent increase in 850 and 1300 nm bandwidth, respectively, for the various launches compared to overfilled. Figure 8 shows

the normalized spread in the mean values reported by the participants for the various launches. Fibers

number 1 through 9 are 62.5 )im fiber and 10 through 15 are 50 |J,m fiber.

300

OFL SO urn 30 urn SMF VCSEL CD Laser FP Laser FIgur* 7 TIA Round Robin 1300 nm Bandwidth Incraas* Launch Condition Figure 8 - TIA Round Robin Interlaboratory Bandwidth Launch Comparison

Although the data had a high level of variability, a couple of observations could be made. First, the best

inter-laboratory agreement was achieved for the overfilled launch. It tended to be the most conservative measure of bandwidth for both the fixed launches and the direct connection measurements (excluding 1300 nm FP lasers). Also it agreed best with the original OFL factory measurements. These results effectively

confirmed the results and conclusions drawn in the late 1970s when the OFL launch was selected for standardization. Second, a significant amount of the bandwidth results exceeded the bandwidth capability

of the test apparatus. This was especially true at 1300 nm and for the restricted launches on 50 fim fiber. However, the direct connection of 1300 nm FP SMF lasers produced consistent bandwidth decreases

relative to overfilled. Bandwidth collapse as great as 40-50 percent was observed. The Gigabit Ethernet

116
02-

Br

Figure 9 - Rising Edge of Digital Signal Showing Plateauing

ONset (fnicfons)

Rgire 10 - DMD at 1300 nm for 62S urn Rber with

Centertine Error

standards group's reaction was to establish link lengths (especially for 1300 nm and 62.5 ^im fiber) based on significantly reduced fiber bandwidths.

Approximately a month or two after the release of these results, another issue arose. Laboratory results using 1300 nm FP SMF lasers and the reconunended length of 62.5 |J.m fiber (standard grade similar to the

installed base) did not achieve the required bit error rate for Gigabit Ethernet operation (8/1/97 Dave Hyer,

DEC). The effect was consistent with different 1300 nm single-mode sources and fibers, and scaled with

fiber length. An example of the rising edge of the digital signal exiting a link is given in Figure 9. The

stair step formed on the rising edge is caused by two discrete delays occurring in the fiber. An example of

the DMD of a fiber showing this effect is given in Figure 10. The DMD is flat over most of the radial extent of the fiber, but shows a large delay error for the lowest order modes on center, such as might be

caused by a center line dip. The OFL bandwidth for this fiber is high since most of the power is carried in the intermediate modes which are well tuned. Thus, this fiber works as desired for overfilling sources such

as LEDs (for which 62.5 |im fiber was designed) but runs into this issue when ultra restricted, single-mode

launches such as those generated by the 1300 nm SMF FP laser are used. Since the original SMF launch

results in 1996 did not show this collapse, the offset of 3 - 7 mm introduced by a MMF connector also is necessary. Such a connector typically is present on the jumper connecting the source to the cable plant.

Faced with an installed base of multimode fiber, whose bandwidth was unknown for such restricted launches, the Gigabit Ethernet standards group chose to control the launch distribution produced by the source. After evaluating various launch specifications and control mechanisms, it was agreed that sources must meet a coupled power ratio (CPR) requirement and that 1300 nm operation would require an offset launch patch cord'.

CPR measurement is described in OFSTP 14^ and consists of taking the ratio of the power coupled into SMF compared to MMF. The requirement ensures that a source launches a power distribution which is fairly overfilling. The offset patch cord achieves a significantly over filling power distribution by

launching from a single-mode fiber into a multimode fiber through a controlled offset created by a special connector. The source must be designed to couple efficiently into the SMF launch fiber. By controlling

the source launch conditions as well as adding a receiver bandwidth requirement and reallocating the jitter budget, agreement was achieved on the link lengths in the current standard and reproduced in Table 2.

Table 2 - Gigabit Ethernet Standard Link Length Specification

Bandwidth

(850/1300 nm)

(MHz-km)

62.5 jim 160 / 500 200/500

50 400/400 500 / 500

850 nm 220 m 275 m 500 m 550 m

1300 nm 550 m 550 m 550 m 550 m

NOTE: 1300 nm operation requires offset patchcord.

117
Current Opportunities

Although the Gigabit Ethernet standard now is finalized, the possibility still exists for developing a methodology for achieving improved system performance over multimode fiber. The laboratory test results over the last few years holds the promise that a reliable solution can be found. Informal discussions have

begun among members of the IEEE 802. 3z regarding longer link lengths especially for short wavelength

operation. Also, the TIA Task Group is still actively continuing its work.

There are three steps to a final solution. First, an effective means must be found to characterize the source power distribution. All experts involved in the decision to use CPR to control the source launch

distribution were not satisfied with the ability of CPR to adequately specify a launch. It did at least ensure

that an ultra restricted launch was avoided. The TIA Task Group is currently investigating Mode Power

Distribution (MPD) measurement^'^" as an alternative. Current consensus is that MPD provides a thorough characterization of a launch distribution. The second step consists of selecting a range of source launch distributions that will achieve improved performance with a range of allowable fiber profiles. It will require interaction between source and fiber manufacturers. DMD modeling of fiber bandwidth for various

launch conditions can be used to determine the acceptable source and fiber characteristics that can

consistently be supplied. The last step is to develop practical test methods to control the source launch

distribution and characterize the fiber performance for the agreed upon restricted launch condition. Like

DMD, MPD has practical limitations for use in specifications. Consequently, it is anticipated that an

alternate can be found similar to CPR.

Acknowledgments

The author would like to express his appreciation to the individuals and organizations that are participating

in the ongoing development work whose contributions are documented herein. The author would

especially like to thank John Schlager and Doug Franzen of NIST for their contributions. NIST, Lucent, Spectran, Corning, Siecor, Honeywell, HP, Cielo / VIXEL, IBM, DEC,

Cabletron, Raytheon, and Unisys.

References

1) K. Giboney, L.B. Aronson, D.E. Lemof, "VCSELs, The Ideal Light Source for Data Nets," IEEE

Spectrum, vol. 35 no. 2, February 1998, pp. 43-53.

2) ISO/IEC 93 14-3, Information Processing Systems - Fiber Distributed Data Interface (FDDI) - part 3: Physical Layer Media Dependent (PMD).

3) Bandwidth model developed for IEEE P802.3z Specification for 802.3 Full Duplex Operation

Supplement to Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method

and Physical Layer Specification for 1000 Mb/s Operation.

4) Mammel & Cohen, 'Numerical Prediction of fiber transmission characteristics from arbitrary refractive-index profiles', Appl. Optics 21 (1982), pp 699-703.

5) Petermann, K., 'Simple Relationship between Differential Mode Delay in Optical Fibres and the deviation from optimum profile', Elec. Lett. 14 (1978), pp 793-4.

6) Olshansky, R., 'Pulse broadening caused by deviation from the optimal index profile'. Applied Optics

15 (1976), pp782.

7) TIA References: Fiber Optic Test Procedures (FOTPs) - TIA-455, Optical Fiber System Test Procedures (OFSTPs) - TIA-526, and Informative Test Methods (ITM) - TIA-TSB62.

8) S.E. Mechels, J.B. Schlager, D.L. Franzen. "High-Resolution Differendal Mode Delay Measurement in Optical Fibers using a Frequency-Domain Phase-Shift Technique," Photonics Technology Letters, vol.

9, no. 6, June 1997, pp. 794-6.

9) L. Raddatz, I.H. White, D.G. Cunningham, M.C. Nowell, "An Experimental and Theoretical Study of

the Offset Launch Technique for the Enhancement of the Bandwidth of Multimode Fiber Links," vol.

16, no. 3, March 1998, pp. 324-31.

10) S. Piazzola, G. DeMarchis, "Analytical Relations Between Modal Power Distribution and Near-Field

Intensity in Graded-Index Fibres," Electronics Letter, 1979, no. 15, pp. 721-2.

11) M. Calzavara, P. DiVita, U. Rossi, "Mode Power Distribution Measurements in Optical Fibres,"

CSELT Rapporti Technici, vol. XI, no. 5, October, 1981, pp. 447-51.

118
Effect of Launch Conditions on Power Penalties in Gigabit Links

Using 62.5|im Core Fibers Operating at Short Wavelength

Petar Pepeljugoski^, John Abbott** and Jim Tatum*^

^IBM TJ Watson Research Center, Yorktown Heights, NY 10598

^Coming Incorporated HPME-01-024, Coming NY 14831

'^Honeywell Microswitch, 830 East Arapaho, Richardson, TX 75081

1. Introduction

An insatiable demand for information bandwidth has led to an explosion of fiber optic networks operating at and

above IGb/s. There is current development effort by several standards bodies, including the IEEE 802. 3z, ATM and

Fibre Channel to develop communications standards for gigabit/sec speeds and beyond. For premises distances, the most cost effective optical networks use multimode fiber. The multimode fiber was designed and specified for light

emitting diodes (LEDs). However, the LEDs can not achieve gigabit/sec speeds, thereby requiring the use of

semiconductor laser diodes. Therefore, there is a need to recharacterize and respecify links for multimode fiber excited by laser diodes. In developing the communications standards, significant differences between the measured

fiber bandwidth using historical overfilled launch conditions and those measured using laser launches have been

found. In most cases, there was a significant increase in the fiber modal bandwidth using the laser launches; however,

there were a few cases in which the bandwidth fell below its rated value. One of the essential requirements of the new

communications standards was the ability to address the ubiquitous 62.5|Lim multi-mode fiber. The rated minimum

bandwidth of Fiber Distributed Data Interface (FDDI) grade fiber is 160MHz km at 850nm, and 500MHz-km at 1300nm. With 1300nm laser excitation, the effective bandwidth of the FDDI 62.5 |im fiber was below specification

in nearly ten percent of the fiber population sampled. An offset launch technique has been described to eliminate the problem [1]. This has proven to be costly, and is not practical for the lower cost, higher volume 850nm solutions.

In this paper, we investigate the performance of the 62.5 |J.m core fiber under various launch conditions. In our study,

both CD lasers and VCSELs were used, with variable spot sizes and numerical apertures, generating numerous

launched power distributions into the optical fiber. Whereas previous analyses of fiber links have used only the fiber bandwidth as the predictor of link performance [1], we use the entire impulse response of the fiber to calculate both

the inter-symbol interference (ISI) and the deterministic jitter (DJ) in the link. Our analysis yields more realistic results than those based on a simple Gaussian response of the fiber transfer function. We find that current IEEE

802. 3z (Gigabit Ethernet) link lengths are quite conservative, and typical system performance will greatly exceed the

specified 220m link length for FDDI fiber.

2. Method of measurements and analysis

The modal power distribution (MPD) in an optical fiber was measured using a technique similar to that described in reference [2]. In our case, a CCD camera was used to capture the laser's intensity profile with a resolution of 0.2 [im.

Additionally, time averaging was employed to reduce the effect of speckle on the near field patterns. The lasers were

driven under normal system parameters. The two dimensional data from the CCD camera was analyzed to obtain the

intensity distribution, mode transfer function (MTF), MPD, and the 95% cumulative power distribution (CPD).

Additionally, the coupled power ratio (CPR) was measured. Circular symmetry was assumed and Figure 1 is a plot of

the mode power distribution as a function of mode group number for each laser.

Once the near field intensity profile in the fiber is known, the mode power distribution is estimated following the procedure of Calzavara et al. and Daido et al. [3,4]. This procedure uses the ratio of the derivative of the near field intensity to that of an overfilled launch. Even if the 2D data was azimuthally averaged, we still found it necessary to radially smooth the intensity data to get a workable mode power distribution. Other approaches to obtain the MPD

include fitting the beam profile with a sum of Gaussian beams with different offsets and beam diameters (whose

individual modal excitations can be calculated exactly) or fitting the beam profile to a sum of modal contributions.

119
Once the modal power distributions of the individual sources are known, they are used with mode delays to numerically model pulse propagation through the optical fiber. The mode delays for the model were determined from

a set of 237 measured index profiles by a perturbation method [5]. Only profiles with overfilled bandwidth that was

predicted to meet FDDI requirements (160/500 MHz km) were used. The mode delays of the individual modes were assumed to be fully coupled and averaged together within each mode group. The transfer function of each optical

fiber for each launch condition is then calculated, taking into account differential modal attenuation [6]. The output

pulse from the optical fiber is represented as a weighted sum of Dirac delta impulses. The transfer function of the

fiber is given by:. H(if)=ZCi^exp(-i27ifXi^). The amount of power and the propagation delay time of the k'*^ mode is given by Cj^^ and X]^ (ns/km) respectively. The 3dB fiber bandwidth is determined by the minimum frequency that

satisfies the condition IH(f)l/IH(0)l=0.5. For the fiber lengths under consideration, we assume without loss of

generality, the mode attenuation is uniform, and there is no mode coupling within the fiber.

300

280

260 1"^ 240 NX

^ 220

szS

? .80

(0 m

160

140

•

• •• •

• •

• —•

•( •

• •

•

•

• • —•

• •

•

•

8 10 12 14 16 18 20 22 Mode Group Number Device number

Figure 1. Mode power distribution for all devices used Figure 2. Minimum fiber bandwidth-distance product for each

device. The inset shows bandwidths for all fibers and devices

3. Correlation of fiber bandwidth and beam profile parameters

The predicted bandwidths are given in Fig. 2. Of the 9006 simulations performed (237 fiber types and 38 sources)

only 0.3% result in the fiber bandwidth falling below the 160MHz km specification of FDDI. For 32 of the 38 sources

the predicted bandwidth was above 160MHz km for all fibers, and for 11 of the 38 sources the predicted bandwidth was above 200MHz km for all fibers. We are interested in characterizing the robust sources which fall in this upper

category.

E^ 20

5) 15 o

Q-

• 95% MP[ PCR

) peak

• .9_ • • • •

< • •

• • •

•

« • <

» •

• -

>

140 150 160 170 180 190 200 210 220 230 Bandwidth [MHz'km]

Figure 3. 95% CPD and peak of MPD as a function of minimum fiber bandwidth.

Mode Group Number

Figure 4. Mode power distribution for devices with bandwidth above 200 MHz km.

In an effort to relate the fiber bandwidth to easily measured parameters, we have examined the bandwidth as a

function of 95% CPD, modal power distribution, full width half maximum (FWHM) and coupled power ratio. Figure

3 is a plot of the minimum modal bandwidth for our devices as a function of the 95% CPD and peak of the MPD.

While each of the measures of fiber intensity distribution have merit, we find that no method is entirely predictive.

120
Our data set shows that the following sets of intensity distribution criteria are sufficient to achieve the minimum

overfill bandwidth (160 MHz km): a) 95% CPD less than 25|im and more than 10(lm; b) the mode power distribution peaks between 10|lm and 25)im; c) coupled power ratio greater than 9dB; d) FWHM between lO^lm and 20|lm. What we found upon further analysis was that the sources whose minimum predicted bandwidth was significantly above

the 160MHz k;m fell into two categories: either the modal power distribution was skewed toward the center with little power on the outside, or the power was skewed to the outside with little power in the center. Figure 4 shows the MPD

for the sources with a minimum predicted bandwidth of more than 200 MHz km.

4. Calculations of Deterministic Jitter and ISI Penalty

The link simulations were performed assuming the worst case parameters (as used in IEEE802.3z) for both the

transmitter and the receiver. We used the Matlab''^'^ based IBM OBST optical link simulator tool, which has built in models for each component in the link (laser, fiber and the receiver). Our simulations used a modification of a linear

laser model which takes into account the duty cycle distortion at the laser output caused by laser turn-on delay. The

receiver was modeled as an 800 MHz 4th order low pass Butterworth filter. The simulator was modified to take into account the fiber model described above. The accuracy of the simulator has been previously verified to match

experimental results [7]. Our simulations used a transmitter with a rise time of 350ps, and deterministic jitter of 65 ps, sending a repeating k28.5 pattern. The numerical results of the simulator included the bandwidth of the optical fiber, the ISI penalty, and the deterministic jitter (DJ) seen by an analog receiver at the end of the fiber link. The DJ was

calculated at the maximum horizontal eye closure measured at the zero point in an AC receiver. The simulations were

done for link lengths of 220m and 300m.

Bandwidth [MHz'krn] Bandwidth [MHz'km]

Figure 5. ISI penalty as a function of the bandwidth distance Figure 6. Comparison of the maximum penalties for fibers 220 m

product. The fibers were 220m long. The inset shows the ISI (circles) and 300 m long (asterisks). Only for bandwidths less penalty for 300m long fibers. than 160 MHz km the penalty difference becomes significant.

Figure 5 is a plot of the ISI penalty as a function of the fiber bandwidth-distance product for a 220m optical link. There are three distinct regions of the curve. The first, for fiber bandwidths less than ~300MHz-km, indicates a unique correlation of the system ISI penalty to the fiber bandwidth. In this region, the system ISI penalty is dominated by the bandwidth of the optical fiber. In the second region, between ~300MHz-km and ~1200MHz-km, the system ISI penalty is determined by a complex interaction of the fiber transfer function, the laser transmitter and the

receiver. The exact ISI penalty is extremely dependent on the details of the input optical pulse. As can be seen from

figure 5, the ISI penalty can range as much as 2dB for a given fiber bandwidth. Clearly the system and fiber response do not have Gaussian shapes. The third region is above ~1200MHz-km where the variance in the ISI penalty

decreases, and the ISI penalty begins to be dominated by the finite bandwidth of the laser transmitter and the receiver. Similar comments can be made about the inset to Figure 5, which is the ISI penalty as a function of fiber bandwidthdistance product for a 300m optical link. Figure 6 shows the maximum ISI penalties at 220m and 300m as a function

of the bandwidth-distance product. For bandwidths greater than 160 MHz km the difference between 220m links and

300m links is less than 0.7 dB, below 160 MHz km the difference between link lengths becomes very significant.

Figure 7 is a plot of the deterministic jitter in the link at the output of the analog section of the receiver as a function

of the fiber bandwidth-distance product. Again, there appears to be three regions of interest, and similar discussion to

121
that for the ISl appHes. The unique relationship between the fiber bandwidth and the deterministic jitter allows for operation of carefully controlled links with fibers having a bandwidth-distance product less than 200MHz-km. As the

link length is increased to 300m, the deterministic jitter increases substantially for fiber bandwidths below

200MHzkm.

Bandwidth [MHz'km] DJ@TP35 after 220m [ps]

Figure 7. Deterministic jitter at the output of the analog section Figure 8. Comparison of deterministic jitter for 220m and

of the receiver, for 220m long fibers. The inset shows the jitter 300m long link. This figure illustrates the complex

for 300m long fibers. Note that 40% increase in fiber lengths dependence of the jitter in the link on the fiber length, almost doubled the jitter. bandwidth and DC unbalance.

Simple link models can often lead to wrong conclusions and conservative estimates. There is quite complex

relationship between jitter and link distance, as illustrated in Figure 8. It appears that it is difficult to predict the jitter for longer links, based on measurement at shorter distances. While the DJ introduces a DC imbalance, after the signal

travels through the fiber, the DJ effects are initially reduced due to ISI and the resulting trend to DC balance. As fiber lengths are further increased, the ISI begins to reintroduce DJ. We find the amount of jitter will depend on many

factors, including the actual pulse shape of the signal and the impulse response of the fiber, not just the bandwidth.

5. Conclusions

In this paper, we investigated the effects of launch conditions on the fiber bandwidth and the link penalties. Our

results show that several parameters may be necessary to fully predict the operation of a fiber optic link. Our results also show that while there is correlation between the bandwidth and the link ISI penalty, this correlation becomes

rather weak for bandwidths exceeding 200 MHz km. Our investigations show not only that the current Gigabit Ethernet lengths can be achieved, but they can be greatly exceeded. Therefore, robust, inexpensive short wavelength solutions, will continue to dominate the gigabit LAN market.

References

[1] Raddatz, L et al., "Influence of Restricted Mode Excitation on Bandwidth of Multimode Fiber Links", IEEE Photonics Technology Letters, vol. 10, no. 4, April 1998, pp. 537-539.

[2] Telecommunication Industry Association OFSTFTTM-3

[3] Calzavara, M. et al, "Mode power distribution measurements in optical fibers," CSELT Technical Report IX (5), pp. 447-

451,(1981).

[4] Daido, Y, et al, "Determination of modal power distribution in graded-index optical waveguides from near field patterns and its application to differential mode attenuation measurement," Applied Optics. 18, pp. 2207-2213, (1979).

[5] Olshansky, R., "Pulse Broadening Caused by Deviations from the Optimal Index Profile", Applied Optics 15(3) March

1976 pp.782-788.

[6] Marcuse, D., Principles of Optical Fiber Measurements. New York: Academic Press, 1981.

[7] Pepeljugoski, P., presentations to the IEEE802.3z MBI study group

122
Improved Performance in Short-Haul Multimode Fibre Systems by

Using Low-Order Launch Conditions

Robert S. Billington', Jerry M. Benson^ Timothy C. E. Jones^ and John C. C. Nelson*

' Institute of Integrated Information Systems, School of Electrical and Electronic Engineering, University of Leeds, Leeds, Yorkshire, LS2 9JT,

r UK.

^ Centre for Quantum Metrology, National Physical Laboratory, Teddington, Middlesex, TWll OLW, UK.

Multimode fibre is increasingly being deployed in local area networks and as part of the control systems in vehicular environments. A significant constraint on the use of this technology in critical systems is the uncertainty in the optical power loss over highly-connected short lengths of fibre which may be subjected to severe bending. In this paper, a summary is presented of launch-dependent effects together with experimental bending loss and

simulated splice loss results. These indicate that the use of restricted launch conditions leads to more predictable losses as well as higher bandwidth

than if a simple overfilling launch is used.

1) INTRODUCTION

Equalisation of optical path lengths in multimode fibre, and consequently maximisation of the fibre bandwidth, can

best be achieved by grading the refractive index of the core to give an almost parabolic index profile [1]. Within

such graded-index fibres, modes may be broadly described as low-order or high-order, where low-order modes

propagate close to the centre of the fibre core and high-order modes have a significant component near the core- cladding interface.

Experimental investigations have shown that by selectively exciting the low-order modes of a graded-index fibre, modal dispersion can be minimised. This has been performed by using a controlled launch system [2] , and

launching from a single mode fibre either directly [3] or via bulk optics [4].

Rayleigh scattering from dopant atoms in the fibre core and imperfections at the core-cladding boundary and onaxis induce mode-dependent attenuation effects with the general tendency that high-order modes suffer greatest

attenuation [5]. The measured attenuation of multimode fibre has been found experimentally to be dependent on

the modal power distribution (MPD) [2]. Results for cabled fibre, connectors and power splitters have also shown

that power loss from low-order modes is less than that from high-order modes [6,7].

In this paper, experimental results are presented which show that low-order modes are significantly more immune

to losses due to macrobending and microbending than high-order modes. In addition, simulation results from a model of a laterally offset butt-coupled joint show that connector losses can be minimised by launching power only

into low-order modes.

The combined effect of all these results is that by using a low-order launch, the bandwidth of a multimode system

will be maximised and the total power loss will be far less dependent on fibre handling and connectorisation than if an overfilling launch is used. It should therefore be easier to calculate the power budget for a system with a smaller

uncertainty than is possible for the case of unrestricted launches.

2) BENDING LOSS

The experimental system shown in figure 1 has been constructed at The National Physical Laboratory; it can be

used to perform controlled launches into multimode fibre and scan the transmitted near-field leaving the far end

for the purposes of modal analysis [8]. The controlled launch system was used to generate launches which

123
predominantly excited low-order modes. By placing the input end of the fibre in direct contact with the LED

source, overfilling launches could also be performed.

Image Plane

Pinhole A1 A2

850 nm

Photodiode

Fixed Loops

Figure 1. Controlled launch and near-field scanning systems.

2.1) Macrobending

A 5 m long sample of 62.5/125 |J.m graded-index fibre was mounted in the controlled launch system and arranged

to include a variable-diameter loop. Transmitted near-field ( TNF ) scans were performed on the intensity profile at the output end of the fibre for a number of loop diameters. The experiment was also repeated using an overfilling

launch to excite the fibre. The measured near-fields for a number of loop diameters with intensity in arbitrary

intensity units ( aiu ) are shown in figure 2. The near-fields were then curve-fitted and converted into modal power

distributions following [7]. Total guided powers in arbitrary power units ( apu ) leaving the fibre were then

calculated from these MPDs and are shown in figure 3.

Figure 2. Transmitted near-field intensity distributions leaving 5 m of 62.5/125 \xm graded-index fibre including a .nngle loop for a) low-order and b) overfilling launch conditions.

124
Power Remaining in 5 m ot 62.5/125 Fibre After a Single Loop of Known Diameter

40

I I

;

I

; r-r-] 4 Figure 3. Calculated optical powers leaving the fibre sample for low-order and overfilling launch conditions with various loop diameters.

It can be seen from figures 2 and 3 that using overfilling launch conditions, whilst

coupling more power into the fibre than a

restricted launch, does lead to considerably

greater sensitivity to bending. The actual power leaving the output end of the fibre with no bends of less than 370 mm diameter was estimated to be approximately 1.5 |a.W for the low-order launch and approximately

30 |J.W for the overfilled launch.

2.2) Microbending

The experimental system illustrated in figure 1 was also used to investigate the effects of microbending on the modal power distribution in a bare 2 m length of 62.5/125 |im graded index fibre. The microbending device

consisted of two aluminium plates lined with copper gauze between which the fibre could be compressed by adding

a weight to the upper plate. The weave of the copper gauze was such that the microbends had a period of between

approximately 0.5 mm and 1.0 mm. Low-order and overfilling launches were again used to excite the fibre and

near-field scans were performed with and without the microbending device in place. The measured near-fields with

and without microbending are shown in figure 4.

3 t

Low-Order Launcfi

0 0 Overfilling Launch

10

2 ?

Diameter of Loop ( mm )

Figure 4. Transmitted near-field intensity distributions leaving aim sample of 62.5/125 (i/n graded- index fibre with and without microbending

for a) low-order and b) overfilling launch conditions.

The attenuafion due to microbending was calculated to be -0.16 dB ±0.16 dB for the low-order launch and 1.52 dB ± 0.20 dB for the overfilling launch. Microbending may be introduced into fibres by the cabling process and

the severity of these imperfections may therefore vary from cable to cable. The restricted launch should avoid

cable-dependent microbending loss by not exciting the high-order and leaky modes which are most easily coupled

out of the fibre by waveguide perturbations.

125
3) SIMULATED CONNECTOR LOSS

Demountable connectors with slight lateral offsets were modelled by calculating the transfer of power between

modes of two identical fibres with zero longitudinal and angular offsets. Three modal power distributions were

passed to the connector model and were then compared with the predicted output modal power distributions to produce the excess loss values shown below in figure 5b. Transmitted near-fields corresponding to the three input

modal power distributions are shown in figure 5a.

Calculated TNFs tor Initial Modal Power Distributions Used in Offset Connector Simulation Calculated Excess Loss of Laterally Offset Connector

Low-Order Launch

Intermediate Launcfi

Overfilling Launch

a)

-V Low-Order Launch

-D Intermediate Launch

-o Overfilling Launch

b)

Lateral Offset (^m).

Figure 5. Simulated excess losses for three different initial power distributions entering a laterally offset connector. Losses due to reflection will be independent of the modal power distribution and have been omitted.

4) CONCLUSIONS

Experimental and simulation results have been presented which show that the use of a restricted launch into a muitimode system will result in more predictable and less sensitive system attenuation than a simple overfilling launch. This, together with the improved bandwidth which can be achieved when restricting the launch conditions,

make this launch attractive for planning short-haul systems where the path of the fibre may need to be flexible or where "worst case" losses need to be considered. Realising the restricted launch in a practical system could be done

by using laser sources or pigtailing LED sources to small-core, low NA fibres which can then be connected to the muitimode system.

5) ACKNOWLEDGEMENTS

This work was funded by EPSRC and The National Physical Laboratory.

6) REFERENCES

[ I ] Gloge, D. , Marcatili, E. A. J. , " Muitimode Theory of Graded-Core Fibers", Bell System Technical Journal 52, pp 1563 - 1578, (1973).

[2] Hackert, M. J. , " Explanation of Launch Condition Choice For GRIN Muitimode Fiber Attenuation and Bandwidth Measurements" , Journal of Lightwave Technology 10, No. 2,pp 125 - 129, (1992).

[3] Haas, Z. , " A Mode-Filtering Scheme for Improvement of the Bandwidth-Distance Product in Muitimode Fiber Systems", Journal of LightwaveTechnology 11, No.7,/5/) 1125 - 1130,(1993).

[4] Franzen, D. L. ei a.l , " Differential Mode Delay Measurements in Muitimode Fibers Using a Frequency Domain Technique with Variable Launch", 4th Optical Fibre Measurement Conference, Teddington, UK, pp 109 - 1 12, (1997).

[5] Olshansky, R. , Oaks, S. , " Differential Mode Attenuation Measurements in Graded-Index Fibers", Applied Optics 17, No. 1 \,pp 1830 - 1835, (1978).

[6] Yang, S. et a.l , " Interlaboratory Comparison of Mode Transition Matrices", Applied Optics 32, No. 30, pp 5997 - 6005, (1993).

[7] Yang, S. , et al. , " Transfer Function Analysis of Measured Transfer Matrices", Applied Optics 28, No. 15, 3148 - 3157, (1989).

[8] Biliington, R. S. et al , "Development of a Muitimode Fibre Simulation Tool for Characterising Attenuation as a Function of Launch

Conditions and Handling" , 4th Optical Fibre Measurement Conference, Teddington, UK, pp 1 13 - 116, (1997).

126
DifTerential Mode Delay Measurements

On Multimode Fibers in the Time and Frequency Domains

J. B. Schlager and D. L. Franzen

National Institute of Standards and Technology, 325 Broadway, Boulder CO 80303

Introduction

Multimode fiber bandwidth is currently measured using an overfilled launch

condition. Such a launch is achieved by uniformly exciting the core and launching with a numerical aperture (NA) that exceeds the NA of the fiber. ^ This launch provides a reproducible way to predict the bandwidth performance of LED-based fiber networks.

New^ high-speed fiber networks, however, take advantage of the faster modulation

capabilities of laser sources like vertical cavity surface emitting lasers (VCSELs) and

Fabry-Perot lasers. These sources typically underfill the available mode volume of the

fiber. In most cases these underfilled or restricted laser launches give bandwidth

performances that meet or exceed the bandwidth performances predicted with overfilled

launches. The specific behavior, however, depends on the differential mode delay

(DMD) of the fiber's refractive index profile. We have developed a frequency-domain

DMD measurement system with a temporal resolution of 0.2 ps. This represents a more

than 50 times improvement over more conventional time-domain DMD measurement

systems." The new system allows accurate DMD measurements on shorter fiber samples,

and the DMD results compare well with those obtained using the time-domain technique.

Frequency Domain Method

The frequency-domain technique employs an integrated optic intensity modulator

that sinusoidally modulates light from an 850 nm or 1300 nm laser source at 1.9 GHz.

Light exiting the single-mode fiber pigtail of the modulator is launched into the

muhimode fiber under test using a butt-coupled launch (< 5 |im separation). The RF

phase of the detected light at the output of the muhimode fiber is compared to a reference

signal derived from the crystal oscillator driving the modulator. Any change in the delay

of the propagation path results in a phase shift, which is detected by a vector vohmeter.

The DMD profile of a fiber is acquired by scanning the single-mode fiber across the core

in 1 |im steps and recording the change in RF phase. Good cleave quality for both the

single-mode launch fiber and the multimode test fiber is important. Fiber end preparation

is monitored using an interferometric microscope to check flatness and perpendicularity.

Comparison with Time Domain Method

To compare DMD measurements made using this technique with the conventional

time-domain method, which measures time-of-flight differences for short optical pulses,'^

we obtained four different 600 m lengths of 62.5/125 jam, graded-index muhimode fiber. Each 600 m sample was cut into three pieces— two 50 m pieces from each end and the

remaining 500 m center piece. The 50 m lengths were measured using the frequencydomain system, and the central 500 m was measured whh the time-domain system. By

127
measuring both ends separately, we were able to detect possible changes in DMD over

the 600 m of fiber. Figures 1-4 show time-domain and frequency-domain DMD data

collected for two fiber samples at the source wavelengths of 850 nm and 1300 nm. Part

(a) of each figure shows the relative delay in pulse waveforms as obtained from a sampling oscilloscope. To obtain these traces, pulses of 150 ps duration at 850 nm and

60 ps at 1300 nm were launched into the test fiber using the same butt-coupled launch

setup as the frequency-domain system. Large area detectors (200 fim diameter at 850 nm

and 150 |j.m at 1300 nm) minimized mode filtering effects but also limited the temporal

resolution of the time-domain system to an impulse response of 200 ps at 850 nm and 115

ps at 1300 nm. Part (b) of each figure shows the DMD data obtained from both the

frequency- and time-domain systems. Good agreement exists for the four cases. Figure 4

shows that when pulse splitting is observed in the time domain, the mean delay of the

waveform (as opposed to the delay of the waveform's peak) compares more favorably

with the frequency-domain result. DMD Length Dependence

Figure 5 illustrates the capability of the frequency-domain technique to capture

changes in DMD over relatively short lengths of fiber. Three 50 m sections, separated by

roughly 500 m, were sampled from a 2.2 km spool of commercial 62.5/125 \im. fiber.

The peak DMD excursion varies from a value of 1 . 1 ps/m in the first section to 3 ps/m in

the third section at a separation of 1070 m. Such variations in DMD may be important

when considering length scaling of bandwidth measurements made on long (-10 km)

fiber samples that ultimately are cut into shorter lengths (-500 m) for application in local area networks.

Acknowledgements

This work was partially supported by the Office of Naval Research and the Space

and Naval Warfare Systems Center.

References

1. "Mode scrambler requirements for overfilled launching conditions to mukimode

fibers," Telecommunications Industry Association Fiber Optic Test Procedure FOTP54.

2. S. E. Mechels, J. B. Schlager, and D. L. Franzen, "High-Resolution DifferentialMode Delay Measurements in Optical Fibers Using a Frequency-Domain Phase-Shift

Technique," Photonics Techiology Letters, vol. 9, pp. 794-796, June 1997.

3. R. Olshansky and S. M. Oaks, "Differential mode delay measurement," mProc. 4"^ Eur. Conf. Optical Communication (ECOC), Genova, Italy, 1978, pp. 128-132.

128
Fig. 1 (a) Relative pulse delay at 850 nm

for 500 m 62.5/125 urn fiber 1.

Fig. 1 (b) DMD at 850 nm from frequency- and timedomain techniques for fiber 1.

-2.0 -1.5 -1.0 -C.5 0,0 0,5 1,0 1,5 2,0 2,5 3,0

Delay (ps/m) Later -

E

•5!Q. Q

Q

2.0-

1.5-

1,0-

0,5 •

00'

-0,5

-1,0-

-1.5 -25 -20 -15

-A— First 50 m Freq. Domain

-o— Last 50 m Freq. Domain - — 500 m Time Domain Mean Delay

-» — 500 m Time Domain Peak Delay

-10 -5 —

I—

10 —

I—

15 —

r—

20 Position (jim)

—

I

25

Fig. 2 (a) Relative |)ulse delay at 1300 nm for 500 m 62.5/125 urn fiber 1.

Fig. 2(b) DMD at 1300 nm from frequency- and timedomain techniques for fiber 1.

toQ.

0)

(AO

Q.

-2.0 -1,5 -1,0 -0 5 0 0 0 5 1 0 1 5 2 0 2.5 3,0 Delay (ps/m) Later

Q

Q

1 5-

10

-0.5

-1 0

-15

-25

—A— 1 st 50 m Freq . Domain —o— Last 50 m Freq. Domain — — 500 m Time Domain Mean Delay —»— 500 m Time Domain Peak Delay —I

-20 —1

-15 ~l-10 -5 0 5 Position (nm)

—

I—

15 —

I—

20 25

Fig. 3 (a) Relative pulse delay at 850 nm for 500

m 62.5/125 nm fiber 2.

Fig. 3 (b) DMD at 850 nm from frequency- and timedomain techniques for fiber 2.

129
Fig. 4 (a) Relative pulse delay at 1300 nm for 500 Fig. 4 (b) DMD at 1300 nm from frequency- and timem 62.5/125 (im fiber 2. domain techniques for fiber 2.

0.5

0.0 H

-0.5

1^ -1.0 Q -1.5 Q

-2.0

-2.5-

-3.0

Section: ——0-50 m —o— 570 - 620 m

1 1 20 - 1 1 70 m

qO u oo 0 oO^ "OOOOOOO

\

0\

-20 -15 -10 -5 0 5 10 15 20

Position (|am)

Fig. 5 DMD at 850 nm for three 50 m sections from a

1.17 km sample of 62.5/125 um multimode fiber.

130
Ultra-high resolution two dimensional differential mode delay

measurements of multimode fibers

Constantine T. Markos, Alfred D. Ducharme, and Gerry Wyntjes

Visidyne, Inc., 10 Corporate Place, South Bedford Street, Burlington, MA 01803

ABSTRACT

The study of Differential Mode Delay '(DMD) in multimode graded-index fibers received

much attention until approximately the mid-1980's' . The number of papers decreased thereafter

most likely as a result of diminished use of multimode fibers in long-distance and high-bandwidth

applications. The recent decision to use multimode fiber for short distance (< 550 m) Gigabit

Ethernet has sparked new interest in the effects of DMD on the bandwidth of multimode fibers^ To

date, only single axis scans of DMD for telecommunication fibers have been published'*. In this

paper we describe a two-dimensional DMD (2D-DMD) measurement for two standard

telecommunication fibers, 50 um and 62.5 um diameters. These measurements were made using a proprietary Digital Phase Processor (DPP) which allowed us to achieve an ultra-high time delay

resolution of 50 femto-seconds.

1. Experimental Setup

The DMD measurement technique is illustrated in Fig. 1 . An intensity modulated 830 nm

laser diode is coupled into a 5 um single-mode fiber (SMF) . The SMF is mounted on an XYZ

translation stage. The multimode fiber (MMF) under test is mounted on a second stationary stage.

A selective mode launch technique is used to excite discrete mode groups in the MMF by raster scanning the SMF over the face of the MMF'.

Laser u - u

Single-Mode Fiber X-Y-Z

Stage

Oscillator Detector Multimode Fiber

Digital Phase Processor Computer

Figure 1. 2D-DMD measurement experimental set-up.

The other end of the MMF is directly coupled onto a photodiode detector which detects the

modulated light after it has passed through the MMF. The reference modulation is compared with

the detected modulation and the phase difference between these signals is calculated by the DPP.

Although different mode groups in a graded-index fiber are designed to have the same mode delay

small perturbations in the optimal index profile are manifested as DMD. These differences are measured and stored in a two-dimensional array in a personal computer.

131
2. Measurements

Figure 2a shows the results for the DMD measurement of a 50 um graded-index MMF fiber as a function of the position. This measurement shows a peak time delay of 18 ps/meter, however,

the average delay near the fiber center is less than 9 ps/meter. In viewing the 2-D plot, the DMD has

a structure showing that the construct of the fiber is asymmetric. The lowest time delay surrounds

the center of the fiber. This suggests that there is a central dip in the index of refraction profile.

Also, there are numerous contours which also suggests that there are ripples in the index profile.

The 62.5 um fiber, Fig. 3a, shows an extremely asymmetric shape in the DMD profile and

has a peak time delay of 27 ps/meter. However, this plot shows that the lowest time delay is at the

center and has an elliptical shape. Like the 50 um fiber, this plot suggests that there are ripples and

large variations in the index profile. Figure 3b shows the effect of the elliptical shape in the X and

Y direction.

3. Conclusion

We have demonstrated an ultra-high resolution DMD in two dimensions. The measurements

show that there are large structures and asymmetries in the time delays. Suggesting that there are

variations, ripples, and central dips in the index of refraction profile. These variations will widen the

impulse response of the MMF which reduces the bandwidth. Two-dimensional DMD provides the

important information needed by fiber manufacturers as feedback for quality control in

manufacturing. This would be a valuable tool for fiber manufacturers since it provides the

information needed to see all the variations in the index profile. One-dimensional scans provide only

limited DMD information.

More measurements are needed to better understand the construct of multi-mode fibers. Due

to lack of funding the XYZ stage used in the experiment was crude, which made raster scanning the

MMF difficult. A computerized motor stage is being obtained to sample smaller spatial increments

of the fiber face. A Mach-Zehnder amplitude modulator will also be used in future measurements

to modulate the amplitude of a laser diode. Modulating the amplitude of the light external to the

laser would eliminate small errors in the wavelength.

Visidyne, Inc. is currently working on a computer simulation of the DMD effect written in

DDL (Interactive Data Language) image processing software. Visidyne, Inc. is in the process of

commercializing this DMD measurement system, and has begun a marketing study to determine the

feasibility and demand for a 2D-DMD test system.

4. References

(1) H. Kobrinski, and G. J. Herskowitz, "Characterization of optical fibers by a differential

excitation technique," A/?/?/;W Optics, Vol. 22, No. 23, Dec. 1983.

(2) D. Gloge, and E. A. J. Marcatili, "Multimode Theory of Graded-Core Fibers," The Bell

System Technical Journal, Vol. 52, No. 9, Nov. 1973, p. 1563.

(3) Gigabit Ethernet accelerating the standard for speed. White Paper, Gigabit Ethernet

Alliance, 1997.

(4) J. B. Schlager, S. E. Mechels, and D. . Franzen, "High resolution differential delay

measurements and bandwidth of multimode fibers," IEEE LEOS, Vol. 2, Nov. 1997.

132
133
62.5 -um QpticBl Fiber

-32 -84 -16 -6 0 B te 24 3S X Poflitlori [micrcajej

Figure 3. DMD measurements for the 62.5 um fiber in (a) 2-D and (b) 1-D.

134
Intermodal Dispersion and Mode Coupling in Perfluorinated

Graded-Index Plastic Optical Fiber

Michael Dueser, W. R. White, and W. A. Reed

Bell Laboratories, Lucent Technologies

600 Mountain Ave. Murray Hill NJ 07974

Tsuyosi Onishi

Asahi Glass Company

1 150 Hazawa-cho, kanagawa-ku, Yokohama-shi Japan

In recent years, there has been great interest in use of plastic optical fiber (POF) for local area network

(LAN) applications. Because polymer materials have

much lower elastic moduli than silica, it is possible to produce plastic fibers with very large cores and still maintain acceptable mechanical properties. Thus,

plastic fibers offer the potential for inexpensive and

robust interconnection as well as simplifiled production

of active system components. Additional impetus has

been provided by dramatic reductions in absorption

loss, made possible by the introduction' of amorphous

perfluorinated polymers for POF applications. Also,

graded-index multimode plastic fibers with non-ideal

index profiles have shown^'^ bandwidths higher than

would be expected from standard calculations of

intermodal dispersion. It has been suggested^ that the

last phenomenon derives from a high degree of mode

coupling in plastic fiber, and there is convincing

evidence that significant mode mixing occurs in step- index plastic fibers produced from standard protonated

polymers, such as polymethyl methacrylate (PMMA). If this result holds true for POF in general, this will be

another important benefit of plastic fiber for LAN

applications. In this paper, we present differential mode

delay (DMD) measurements which strongly suggest

mode mixing occurs on very short length scales in perfluorinated plastic optical fiber. To our knowledge,

this work represents the first DMD measurements

performed on perfluorinated POF. Also, since we

measure fiber with much lower loss than that obtained

in conventional POF, we may examine intermodal

dispersion at length scales longer than those previously

accessible.

In a differential mode delay experiment'*, one

excites a multimode fiber with a pulsed source having a spot size much smaller than the fiber diameter. By

varying the position at which the input spot impinges

on the endface of the fiber, one changes the relative excitation of different modes. In particular, moving the launch position away from the center of the fiber core

redistributes power from low-order modes into higherorder modes. Thus, by measuring the time required for the input pulse to propagate through the fiber as a

function of the launch position, one may probe

intermodal dispersion in the fiber. Fig.l describes an

apparatus which we have constructed to perform DMD

measurements of large core plastic fiber. An 820nm

diode laser generates pulses of width 200ps with a

repetition rate of IMHz, output through a pigtailed

5^m diameter 0.12 N.A. singlemode fiber. We mount

the singlemode output in an actuator which performs a two-dimensional scan over the surface of the multimode fiber of interest. At the output of the multimode fiber, we use a combination of a graded

index rod lens and a plano-convex lens to focus the output pulse onto the 200 \im active area of a Hamamatsu C4258 detector. The output of this detector connects directly to the sampling head (model SD-22)

of a Tektronix CSA 803 digital oscilloscope, which

triggers from the pulsed source. With this instrument, we have examined

perfluorinated graded-index plastic optical fiber produced^ by the Asahi Glass Company. This fiber had

outer diameter 350 |xm, with a core diameter of

approximately 140 ^m. The index profile is approximately parabolic, with a core refractive index of

1.355, and a cladding refractive index of 1.342. At 820

nm, this fiber has a loss of 60 dB/km. To prepare the

fiber for measurement, we polish the fiber ends by

dipping^ in a perfluorinated solvent. Figure 2 shows typical pulses measured with

various launch positions after propagation through a

1 1 8m length of the POF. For a launch position at the

center of the core, the output pulse shows only a slight

135
Pulsed

Laser Diode 820 nm I

Trigger signal

Digital Oscilloscope

Figure 1 Schematic diagram of experimental apparatus

broadening compared to that measured for a very short

(1 m) length. (Pulse heights have been scaled to compensate the ~7 dB loss occurring in the fiber.) As

we move the input position away from the core center, the pulse is slightly broadened and attenuated, and the broadening and attenuation becomes more pronounced

as the oflfeet approaches the core radius. More

importantly, the centroid time delay of the pulse

decreases with offset, as a result of intermodal

dispersion. In this particular fiber, the centroid delay

decreases with increasing offset, indicating a larger

propagation velocity for higher-order modes. Such a

result is expected for fibers with an overcompensated

index profile. To map the dependence of pulse delay

and attenuation on launch position, we systematically scan the pulsed source over the face of the fiber in 15|im steps, and measure the integrated intensity and

centroid time delay of the output pulse for each launch

position. Since we expect cladding modes to be strongly attenuated by the coiling of the fiber during

measurement, we minimize the influence of these modes by measuring only under launch conditions

where the integrated intensity of the output pulse is at least 10% of that measured for a launch into the center of the fiber core. We have verified that this cutoff

criterion results in a constant scan area for all fiber lengths measured, indicating that the measurements

presented here are not significantly influenced by

cladding modes launched at the input.

Fig.3 shows a typical result of pulse delay

measurements in a scan over the fiber. In this plot, each

contour represents a range of pulse delays of 40 ps, with the darkest contour representing the longest delay

range. In this measurement, one may readily observe

that the pulse delay has a slight azimuthal asymmetry

with respect to the launch position. This pattern

changes only slightly with fiber length, so presumably

the observed asymmetry arises from a similar

asymmetry in the index profile. Also, the position

4-

— 3-

' I ' I I ' 1 A Core center

j

' 1 ' 1

1 \ Offset 30 nm / \

1 \ Offset 60 [iin /

1 1 ir^put I \

/ \ --y

0 500 1000 1500 2000 2500 3000

time [ps]

Figure 2 Typical output and input pulses

dependence of the pulse delay increases very rapidly near the periphery of the core. Since the source spot has

finite size, this position dependence probably accounts

for the broadening observed in pulses injected near the

periphery.

While the characteristic pattern of delays shown in Fig. 3 does not change with fiber length, the magnitude

of the variations naturally decreases with length. To

make a simple metric of the magnitude of the

variations, we have calculated the standard deviation of

the delays measured at the 50 measurement points nearest to the core center, corresponding to a sampling

area slightly smaller than that in Fig.3. As we show in Fig.4, this RMS delay variation, a, depends on fiber length in a way that is reasonably well described by a power law, a oc L^^^. From this result alone, we may

infer clear evidence of coupling between modes, since

uncoupled modes would show delay variations proportional to the first power of length, which is clearly inconsistent with our observations. Indeed, the observed length dependence is quite close to the dependence cr x L*'^ expected from a diflfiisive theory^

of mode coupling. It is worth noting that the length

dependence observed here is identical to that seen^ in pulse broadening measurements of step-index PMMA

POF under overfilled launch conditions. Presumably, we would observe linear intermodal dispersion at lengths below the characteristic length for mode

coupling, but this characteristic length appears to be

well below the shortest length (20 m) measured in these experiments.

In plastic fibers of the type discussed here, many

possible sources of mode coupling exist. As the fiber cools from the melt state during the draw process, thermally excited fluctuations of density and

composition, as well as polymer orientation, become

frozen into the fiber, resulting in very short length scale

136
«0-

-60 -30 0 30 60 X Axis Launch Position (nm)

Figure 3 Variation of pulse delay with launcti position Darker contours

represent longer delay, and the spacing tietween contour levels is 40 ps

inhomogeneities. Our measurements^ of light scattering

in bulk samples with composition similar to that of the core show that these intrinsic fluctuations produce a Rayleigh scattering intensity corresponding to a loss of

-10 dB/km. In addition to these fluctuations, one might

expect extrinsic sources of scattering, such as voids,

cracks, microbends, index profile variations, and

diameter variations introduced during the draw process. Indeed, since the measured loss is roughly 60 dB/km,

and we know that the absorption loss is negligible in these perfluorinated materials, we may be sure that

extrinsic sources of scattering dominate the loss in this

fiber. If mode coupling resulted primarily from the

intrinsic scattering sources, then the characteristic length on which mode mixing occurred would be larger

than that on which those fluctuations scattered light

from the fiber, since Rayleigh scattering is isotropic

and the low NA would allow only a small fraction of

the scattered light to be recaptured in core modes.

Thus, intrinsic fluctuations would produce mode

mixing only after hundreds of meters, so we may

conclude that extrinsic sources are primarily

responsible for the mode mixing observed here. This

result is expected, since extrinsic scattering sources may exist on larger length scales, resulting in strongly

forward scattering, and more efficient mode mixing for a given rate of attenuation.

In conclusion, we have presented differential mode

delay measurements of perfluorinated graded-index

plastic optical fibers, and these measurements have

shown for the first time that intermodal dispersion

scales roughly as the square root of length in these

fibers. This result strongly suggests that the

characteristic length for mode coupling is less than 20

meters, so that mode coupling greatly improves the bandwidth of perfluorinated POF over lengths

appropriate for LAN applications. Since precise control of the index profile is difficult in plastic fibers, this

- T 1 1 ,

A.- A.'

- 'j^

.'

•

'

'

' ^ ^ , 0.57 a ~ L

10 20 40 60 80 100 200

Fiber length (m)

Figure 4 Dependence of delay variation on fiber length

outcome also has positive implications for the prospect

of large scale production of POF with high bandwidth.

Finally, since extrinsic scattering sources appear to dominate the mode coupling, our measurements

suggest the possibility of tailoring these extrinsic inhomogeneities to produce rapid mode coupling with

minimal loss.

Acknowlegements

The authors acknowledge many useful discussions

with L. L. Blyler, A. F. Garito, C. Koeppen, Y. Koike, X. Quan, and R. Shi.

References

[1] E. Nihei, T. Ishigure, N. Tanio, and Y. Koike,

lEICE Trans. Electron., V.E80-C, 1 17 (1997).

[2] T. Ishigure, A. Horibe, E. Nihei, and Y. Koike, J. ofLightwave Tech., v. 13, 1686 (1995).

[3] G. Jiang, R. F. Shi, and A. F. Garito, IEEE

Photonics Tech. Lett., v. 9, 1128 (1997).

[4] Dietrich Marcuse, "Principles of Optical Fiber

Measurements", Academic Press, New York (1981).

[5] H. Murofushi, Proc. Fifth Int. Conf Plastic Optical

Fibers and Applications, p. 17, (1996).

[6] Noriyuki Yoshihara, Proc. Sixth Int ConfPlastic Optical Fibers andApplications, p. 27, (1997).

[7] D. Gloge, BellSyst. Tech. J., v.51, 1767 (1972).

[8] W. R. White, and Pierre Wiltzius, to be published.

137
i

138
Measurement of Skew in GI MM optical ribbon fibers

Masateru Tadakuma Osamu Aso Shu Namiki

Optical transmission systems group wp-project team R&D division, Furulowa Electric Co., Ltd.

6, Yawatakaigan-dori, Ichihara, Chiba, 290-8555 Japan

phone:+8 1-436-42-1724 fax:+81-436-42-9340 e-mail:tadakuma@ch.furukawa.co.jp

Abstract

Since the skew tolerated in parallel interconnections is about one to several ps/m, it is necessary to measure

the ribbon fibers skew with precision of the order of O-.lps/m. We have realized two methods to measure skews of multimode (MM) ribbon fibers. One is pulse delay method, and the other is phase shift method. In the pulse delay method , an optical short pulse source which generates Gaussian pulses with 130ps pulse width was used and the difference of the delay time (skew) between a reference fiber and others in the ribbon fibers was measured. We could achieve the measurement standard deviation within 7ps for a 100m ribbon fiber (0.07ps/m). In the phase shift method, we measured skews using a network-analyzer and optimized the measurement conditions. It enabled us to measure skews with the standard deviation of 0.04ps/m.

1. Introduction

Recently, in the fields such as large capacity switching systems and massively parallel processors, highspeed

synchronous parallel frame-to-frame and board-to-board data transmissions tend to limit the degree of freedom in designing the system structure. Optical interconnections using optical fiber arrays are expected to

alleviate the system- to-system interconnection bottleneck. Components of optical parallel interconnections include transmitter and receiver modules which carry laser diode (LD) and photodiode (PD) arrays,

respectively, and multichannel optical ribbon fibers which connect the transmitters to the receivers. Low cost

fiber-optic links use multimode (MM) fibers with either a 50 or 62.5M-m diameter graded index (GI) core, as

this results in large optical alignment tolerances and more robust fiber splices and connector components. In

this transmission system, the delay time difference (skew) of signals between optical fibers in ribbon fibers becomes an important factor. The ribbon fibers skew limits the longest transmittable length. Generally, the skew tolerated in parallel interconnections is about one to several ps/m. Therefore, it is necessary to measure

the ribbon fibers skew with precision of the order of O.lps/m. In this report, we have developed the skew

measurement methods of the multichannel multimode ribbon fibers.

2. Skew measurement method

Similar to chromatic dispersion measurement, skews can be measured by the the pulse delay method and

phase shift method . 1) Pulse delay method

In the pulse delay method, the optical short pulse launched into the fiber under measurement and the delay

time difference is determined from the propagation time of the pulse. An optical pulse from a laser diode (LD)

modulated with a pulse generator is injected into the fiber. The light emitted from the fiber is observed by an

optical sampling-oscilloscope. Skews are calculated by comparing between observed output pulse positions.

2) Phase shift method

In the phase shift method, sinusoidally modulated light is launched into the optical fiber, and the light emitted from the fiber is converted into an electrical signal by a photodetector. The phase angle difference of this

output light to a reference signal is measured for each fiber and determines the ribbon fibers skew |1| 12].

3. Study of the pulse delay method

We have developed a measurement system based on the pulse delay method and reduced measurement

deviations. The improved experimental setup is shown in Fig.l.An electrical signal generated by a pulse

139
generator is converted into an optical pulse via a LD and separated into two ports by a coupler. Pulses propagating along a reference fiber and another fiber under measurement are recoupled by the coupler at the output. Both ends of the test ribbon fibers are bared and cut square, and set on fiber-positioning holes of mechanically transferable (MT) ferrules. These MT ferrule-endfaces contact with MT ferrule endfaces of fanout devices which convert a twelve-fibers ribbon to twelve individual fibers. We measured the arrival time using the optical sampling oscilloscope. The delay time of the pulse propagating along the reference fiber is taken as the reference time, skew is decided as the difference between the reference time and the delay time of

the fiber under measurement. For a multi-channel fiber ribbon, the reference fiber is fixed while the other

fibers are set as the fiber under measurement in turn.

FC connector^ Panout device FC connector 850nm \ ^ ^ / Sampling

Pulse light Pulse generator source

Coupler ^

Measured ribbon fibers MT connector mT connector

^ / oscilloscope

O

^

Coupler

Trigger signal

Fig.l The improved measurement system

In the measurement system shown in Fig.l, pulses from two fibers happened to overlap each other at the optical sampling oscilloscope and it was difficult to distinguish one pulse from the other. Therefore we

prepared different lengths in the output coupler's ports. In the pulse delay method, the optical pulse source

condition is important. Factors necessary for the skew measurement can be listed as follows. 1. Stability of pulse profile 2.Narrow pulse width The reason of the first factor is because the delay time is decided by

fitting observed pulse profile with the most appropriate function. By the second factor, it is expected to decrease observational error at pulses' position. To meet these factors, the vertical cavity surface emitting

laser (VCSEL) was used and modulated with a 400ps width signal near lasing threshold. The short pulse

forms a Gaussian profile with the wavelength of 850nm, the pulse width of 130ps and the repetition

frequency of lOOMHz. We fitted pulses with Gaussian function and decided the center position of the pulses on the time axis. The skew is determined thereby. Fig.2 shows the skew obtained from this method.

"direct connection between couplers

"propagating along ribbon fibers I delay time difference J (skew)

"nHer'IengtliT difference : of roiiler p(jrt'^

(i^. propagating

along-d reference

,fii>.ex,

JiV: •

! ; I ; propagating:

P

; V along a

" i-measured"—.

Time (Ins/div.)

Fig.2 Skew measurement from observed pulse profile

140
4. Study of the phase shift method

In the phase shift method, to measure the signal phase emitted from the fiber, a network-analyzer is used. The

phase value measured by the network-analyzer is the phase difference between the original signal generated

from the analyzer and the signal through the fiber. For small measurement deviation ,we have selected a

better light source out of two, as described below, and investigated the network-analyzer settings. The

experimental setup is shown in Fig.3.

Light source 850nm

Network-analyzer

oo

0/E converter

' Fanout device

Q—yu

Table. 1 Skew meansurement conditions

Measured ribbon fibers MT connector MT connector

Frequency (Hz) 800M

Average times 10

Measurement time 12s/lfiber

(2.5minutes/l 2ch-ribbon fibers)

Fig.3 The phase shift measurement system

In the setup shown in Fig.3, we removed the ribbon fibers and formed a short circuit by MT connectors of input and output sides. We then compared measurement deviations using a multimode LD with those and

using a VCSEL. In the same condition, we calculated deviations of measured values for ten times. The

measurement deviation with the VCSEL was about 1.6ps smaller than that with the multimode LD. Since many different wavelength axial-modes oscillate in a multimode LD, slightly different velocities of those axial-modes might increase measurement deviation. Furthermore, the measurement condition of the networkanalyzer has been optimized. It is considered that factors which influence measurement accuracy of the network-analyzer are modulation frequency, sweep time (resolution bandwidth), and average times. Since delay time per phase angle is reduced by raising the modulation frequency, it becomes possible to measure

with better resolution. But the frequency is limited by the modulation bands of the light source and the

receiver. By extending the sweep time and increasing average times, it enables us to bring out a good

performance of the analyzer, but measurement time becomes long. During long time measurement, the temperature changes of the test ribbon fibers occur and cause the deviation of measurement phase angles, true skew value is not well defined. In so doing, we have optimized and found the measurement conditions which

bring the best performance out of this setup. The result is shown in Table. 1.

5. Skew measurement deviations

12ch MT ferrules are used at both ends of the test ribbon fibers to connect with fanout devices in both the pulse delay method and the phase shift method. In Fig.3, to evaluate measurement deviations due to changes

in alignment conditions of MT ferrules, we investigated skew value deviations for 10 reconnections while

both ends of the test ribbon fibers were cut square and set on fiber-positioning holes of MT ferrules at each

reconnection. To eliminate the effect of changes of fiber length by temperature deviation, the test ribbon fibers were placed into a thermal chamber to maintain constant temperature at 25'C. The phase shift method was

used. The result is shown in Table. 2. Table. 2 Skew value deviation of a sample fiber for 10 reconnections

Fiber length-lOOm

Fiber number inl 2ch-ribbon fibers No.6 No.l2

Standard deviation of measured value (ps) 1.224 1.799

It is considered that the measurement deviations due to reconnections are jointly caused by fluctuations of the

light source and the network-analyzer, by the variation in bared and cut fibers lengths of both the test ribbon

fibers ends, and by delay time difference between propagating modes along fibers. The maximum variation in

141
bared and cut fibers lengths of one end of the test ribbon fibers is about SOM-m (the corresponding

propagation time is about 0.2 5ps). At joints between ribbon fibers under measurement and fanout devices,

fiber cores face each other. If the fanout fiber is joined to the fiber in the ribbon but with axial misalignment, the light will be coupled to different propagation modes, and different modes will propagate at slightly different velocities along the fiber, and measured skew values will vary.

6. Measurement results of 12-channel GI MM ribbon fibers skew

Skews of r2-channel ribbon fibers constituted of Gl multimode fibers with core diameter 62.5M.m were measured by both the pulse delay method and the phase shift method. For each fiber of the ribbon, we

calculated the average of the delay time differences between the reference fiber and the fiber under

measurement, for five times. The reference fiber is fiber No.l in Fig.4. The measured ribbon fibers are 100m

long. In the pulse delay method.the standard deviation of measured values for five times was 0.068ps/m,

while in the phase shift method, the standard deviation was 0.042ps/m. The skew across the 12-channel

ribbon fibers was 196.1ps/100m by the pulse delay method, 185.1ps/100m by the phase shift method.

150.0

100.0

2 50.00 cI 0.000

-50.00

-100.0 123456789 10 11 12

Fiber No.

Fig.4 Skew of the 12-channel ribbon fibers

Table. 3 Skew of measured 12ch ribbon fibers

Fiber core/outer diameter

Phase shift method (ps/m) Pulse delay method (ps/m)

Skew Standard deviation Skew Standard deviation 62.5/125//m 1.851 0.042 1.961 0.068

The difference of skew values between the two skew methods was lips. It is considered that the difference is caused by the launch and environmental conditions of the ribbon and fanout devices.

7. Conclusion

We have developed two methods to measure skews of GI multimode ribbon fibers. One method is the pulse

delay method and the other is the phase shift method. In the pulse delay method, observed pulse profiles were

fitted by a Gaussian function to decide accurate delay time. The standard deviation of the measurement was

6.8ps for 100m (0.068ps/m) .In the phase shift method, by the optimization of measurement conditions, it enabled us to measure with the standard deviation of 0.042ps/m. Both methods developed in this work have

achieved sufficient measurement precision.

8. References

|1] R. Matsuoka et al., "Measuring method for Ribbon fiber Skew" Tech. Digest-Symposium on Optical Fibre Measurements 1992,ed. by G.W.Day and D.L.Franzen,pp. 41-44 1992.

\2\ A. J.Barlow et al, "Measurement of ultra-low skew in ribbon fibers" Tech. Digest-Optical Fibre

Measurement Conference 1997

1 I I

1 A . . ^. . phase metho ;l

fi

i

i

i i

142
Gratings for WDM Systems -

Review of Applications and Critical Measurements

Peter A. Krug, John Canning and Dmitrii Yu. Stepanov

Australian Photonics Cooperative Research Centre, OFTC

101 National Innovation Centre

Australian Technology Park

Eveleigh, NSW 1430

Australia

1. INTRODUCTION

Since their invention less than a decade ago, side-written optical fibre Bragg gratings (FBGs) [1] have

progressed rapidly from laboratory curiosities to commercially important devices in optical communications and

sensing. With the advent of wavelength division multiplexing (WDM) in optical communication networks, the role of FBGs has become crucial. The stringent demands on performance of FBGs in a variety of WDM applications has

tested the ingenuity and skill of those who manufacture them, and have called for increasingly complex grating

structures to be realised. Consequently, the development of techniques for the measurement of grating performance

and structure has become a field of research and development in its own right.

In this paper, we review the important applications of fibre Bragg gratings, and their close relatives, the socalled "long period" cladding mode coupling gratings [2], and describe some of the techniques used to quantify their optical response and physical structure.

2. APPLICATIONS

2.1 Filtering

Because of the large wave vector mismatch between the incident and reflected waves, a simple Bragg

grating is essentially a narrow band device. For highly reflecting gratings produced by periodic refractive index

changes of order 10'\ reflection bandwidths are of the order Inm or less. Such spectral widths typically correspond

to a single channel in a WDM communications system. To obtain a filter response covering more than a single

channel, a number of modifications of the grating structure can be employed. These grating structures, several of

which will be discussed below, include grating structures modified by sampling [3], chirping [4], concatenation, or overwriting of multiple gratings in a length of fibre. An alternative approach is to use a long period grating (LPG)

which couples a core-guided mode to a co-propagating cladding mode [2]. Because the difference in wave vectors between the incident and resonantly coupled waves is much smaller than in a FBG, the resonant bandwidth is correspondingly larger. Typical resonant bandwidths of 5nm or more can easily be achieved [5]. Some examples of

filtering applications in WDM systems are presented below.

Gain equalisation in nonlinear wavelength conversion. Morgan et al [6] have used a filter comprising 9

concatenated FBGs to equalise the wavelength converted signals generated by four wave mixing in a semiconductor

optical amplifier. Without equalisation, the conversion efficiency of the converter varies by up to 13dB over the 10

wavelength channels, spaced lOOGHz apart. With equalisation, the channel-to-channel variation in conversion

efficiency was reduced to ±1 .5dB.

Gain flattening in an erbium doped fibre amplifier. Wysocki et al [7] have shown that almost perfect

flattening of the gain of an erbium doped fibre amplifier (EDFA) can be achieved by use of a filter comprising

several LPGs whose strengths, centre wavelengths and spectral widths were optimised by computer modelling. The

143
resultant net gain was flat to within ±0.5dB over a wavelength range of 1528. Inm to 1568.1nm. A major advantage

of LPGs over FBGs in this application is that while FBGs reflect the resonantly coupled light, LPGs couple the

rejected light into the cladding of the fibre, where it can be easily dumped, for example by use of an absorbing

coating on the fibre.

Tuneable equalisation ofEDFA gain. Kim et al [8] have used acoustic standing waves in a length of fibre to produce tuneable LPGs. The periodic microbending losses induced in the fibre by the acoustic wave, rather than

ultraviolet light induced index changes, are responsible for the coupling to cladding modes. By simultaneously

exciting the fibre with several radio frequency sinusoidal acoustic signals, the researchers were able to demonstrate

complex, tuneable filtering profiles. Such rapidly tuneable filters are likely to find immediate application in actively

controlling the gain profile of EDFAs.

2.2 Multiplexing and Demultiplexing

Simple demultiplexing. Mizrahi et al [9] demonstrated 4 channel demultiplexing using pairs of nonspectrally overlapping gratings, arranged such that each grating pair transmitted one WDM channel (in the gap

between the two reflection bands) and reflected all other WDM channels. A 1 x 4 fiised fibre splitter with one such

grating pair filter on each output arm was able to separate the channels, with a selectivity of better than 35dB.

Add-drop multiplexing. Giles and Mizrahi [10] demonstrated that a FBG placed between two 3-port optical

circulators can be used to drop an optical signal whose wavelength is reflected by the grating, and insert a new

signal at the same wavelength. Because the insertion loss of optical circulators is less than IdB port-to-port, the total insertion loss for any of the three signal paths (through, add and drop) is less than 2dB.

Using an array of four concatenated FBGs, each controlled by a piezoelectric actuator, Quetel et al [11]

demonstrated a programmable add/drop multiplexer. The selection of channels to be dropped was made by dc

voltages applied to the piezoelectric actuators. Switching times of 30|is were achieved.

2.3 Dispersion Compensation

Since the first experimental demonstration of dispersion compensation using a chirped fibre Bragg grating

in 1994 [12], the chirped FBG has shown itself to be a serious competitor with dispersion compensating fibre (DCF)

for WDM transmission at 1.5|im in standard fibre. Its advantages over DCF include its negligible polarisation mode

dispersion (PMD), low insertion loss, compacmess and, potentially, lower cost. However, significant ingenuity has

been necessary to fabricate chirped fibre Bragg gratings with bandwidths sufficiently large to cover four or more WDM channels. Ouellette et al [13] showed that a chirped FBG that is also periodically sampled can achieve

dispersion compensation simultaneously at multiple signal wavelengths. Such gratings can be fabricated by

sampling a chirped grating (for example by introducing spatially periodic phase steps [13], or equivalently, by

overwriting chirped gratings centred at the signal wavelengths, in a single length of fibre. Chirped gratings able to compensate for 50km worth of group velocity dispersion in 8 channels have recently been reported [14]. Alternatively, continuously chirped gratings up to one metre and longer, with bandwidths covering a substantial

fraction of the EDFA gain spectrum have been reported [15].

An alternative way to use fibre Bragg gratings for group velocity dispersion compensation is to use the dispersion of a grating in transmission, just outside its reflection band. Eggleton et al [16] have demonstrated use in transmission of a semi-apodised, unchirped FBG to compensate for dispersion of 72km of standard fibre at 1544rmi.

Several techniques have been reported for writing chirped gratings for dispersion compensation, including

translation of the fibre or the phase mask or rotation of interferometer mirrors during ultraviolet exposure;

deformation of the fibre following grating inscription [4]; concatenation of short, uniform gratings centred at increasing or decreasing wavelength. All such gratings suffer from small imperfections which produce noise-like

ripples in the group delay spectrum. Minimisation of these ripples has been a major focus of groups working on

chirped gratings. The measurement of group delay, therefore, has been of particular importance. This is discussed in section 3.2.

144
2.4 Optical Sources

FBGs have been used in many ways as resonator elements for fibre and semiconductor lasers, and to

stabilise the wavelengths of various lasers. An extensive review of applications of fibre gratings in fibre lasers and

amplifiers was recently published by Archambault and Grubb [17]. A few examples are presented below.

Wavelength stabilisation of semiconductor lasers. Morton et al [18] used a chirped FBG in a lensed fibre

pigtail as a wavelength selective resonator element in a modelocked hybrid semiconductor laser. The laser was only

stable when the chirp direction was such that the grating period increased with increasing distance from the laser chip.

Multi-wavelength fibre lasers. Chow et al [19] used both a chirped FBG pair [20] and a sampled FBG to produce simultaneous lasing on several lines in an erbium doped fibre ring laser. They obtained simultaneous

emission on eight lines when the erbium doped fibre was cooled to 77K, to suppress the Doppler broadening of the emitting transition.

Step-tuneable fibre laser. Ibsen et al [21] used a pair of sampled FBGs as resonator elements in a linear cavity erbium doped fibre laser. The sampling periods of the two FBGs were different, and selected such that only one resonant peak of each grating was coijicident with a resonant peak of the other grating at any time. Small strain applied to one of the gratings changed the wavelength of the coincident resonance so as to cause the output of the

laser to jump successively to different peaks in the grating response spectra. The laser tuned in approximately 2nm

steps over a wavelength range 1532 to 1549nm with a strain of only 0.14% applied to one of the gratings.

Multi-order fibre Raman laser. Grubb et al [22] used nested pairs of FBGs, each pair resonant with one

order of Raman emission from an optically pumped germanosilicate fibre. 1.7W of output at 1480nm, being the fifth order of Raman emission of the 1 175nm wavelength pump, was emitted with a slope conversion efficiency of 46%.

A similar device, configured as an amplifier at ISlOnm, has produced a gain of 40dB and an output power of

+24dBm [23].

3. GRATING MEASUREMENTS

The optical response of a grating can, in principle, be obtained from perfect knowledge of its structure. Conversely, full information about a grating's structure can be calculated by appropriate inverse scattering calculations based on its reflection, transmission, loss and phase spectra [24]. Below, we present recent techniques

for characterisation of grating structures and optical responses of gratings.

3.1 Reflection and Transmission Spectra

Power reflection and transmission spectra are most rapidly measured using a broadband source, such as a superluminescent light emitting diode or a superfluorescent doped fibre, together with an optical spectrum analyser

(OSA). The minimum resolvable spectral feature is > 0.07nm for most commercial OSAs, although instruments

incorporating Fabry-Perot etalons, capable of resolving 0.00 Inm are now becoming available. However,

measurements at such high resolutions are very slow, because of broadband source intensity limitations. For faster measurements at picometre resolutions, the universally accepted spectrometer comprises a step-tuneable external

cavity semiconductor laser and an optical power meter. The trade-off between the two schemes involves speed and

resolution. While broadband sources and OSAs can capture a spectrum in one or a few seconds, the tuneable laser and power meter can take minutes to tens of minutes or more, depending on the required resolution and spectral range. An additional limitation, especially with the scanning source method, is the relative lack of absolute wavelength calibration. Use of an optical wavemeter can greatly improve wavelength calibration (and prevent

inaccuracies due to source mode hopping), at the expense of significant cost and increased measurement duration.

Some spectral features, such as the narrow transmission band in a n/4 phase shifted grating [25] can be 1pm

wide or less. Stepanov et al [26] have demonstrated an ultrahigh resolution variant of the tuneable wavelength

scanning method to measure grating spectra. They report using RF modulation of an optical source to produce a

145
tuneable sideband, which is used as the probe. The researchers achieved a measurement resolution of lOOkHz

(approximately lO'^nm), which is limited by the laser linewidth. The time required for a measurement is of the order

of one millisecond.

Measurement of grating sidebands and other weak features is limited by the dynamic range of the measurement technique. The finite selectivity of an OSA and the presence of broadband amplified spontaneous

emission in narrowband tuneable sources limit detectability of such weak features to about -30dB. Use of a narrowband tuneable filter can allow measurement to about -60dB.

3.2 Group Velocity Dispersion

The most commonly employed technique to measure the group velocity dispersion is based on a technique

of Ryu et al [27]. The output of a narrowband, tuneable optical source is externally RF modulated to produce

sidebands. The modulated signal in reflected from (or transmitted through) the FBG, and the phase difference

between sidebands is measured by a vector voltmeter or network analyser. The phase difference is a measure of the average dispersion of the grating over a range given by the modulation frequency. Tuning the wavelength of the source enables the dispersion to be measured as a function of wavelength. Dispersion measurement resolution better than 5ps/nm, which is essential for resolving the group delay ripples which plague chirped FBG dispersion

compensators, is easily achieved by this method.

Ouellette and Stepanov [28] reported a novel measurement technique based on measuring the reflected light

in two orthogonal polarisation modes. Use of a broadband source and optical spectrum analyser enables the measurement of the group velocity delay spectrum in a very short time, using only a very simple optical setup.

3.3 Spatial Profile

Although measurement of the transmission, reflection, loss and delay spectra can, in principle, enable one

to calculate the spatial structure of a grating, any noise or inaccuracy in the measured spectra will lead to ambiguities in solutions of the inverse scattering calculation. The problem is compounded in the case of complex

grating designs. Therefore, it is necessary to obtain direct measurements of grating structure. A number of

techniques have been reported.

Index modulation depth. Krug et al [29] focussed a visible HeNe laser beam, angled at the first order Bragg

diffraction angle, onto the core of a fibre containing a grating. The fibre grating diffracted a fraction of the incident beam, with a diffraction efficiency proportional to the square of the index modulation depth An. By scanning the probing beam along the grating in the axial (r) direction, and recording the intensity of the diffracted light beam as a

function of z, they obtained a direct measure of the index profile, An(z). Ouellette et al [30] used this method to characterise both a fibre grating, and the phase mask which had been used to fabricate it.

Side scatter measurement ofgrating resonant structure. Canning et al [31] have developed a powerful FBG

characterisation method based on spatially resolved measurement of light incoherently scattered within the grating,

and emitted from the side of the fibre. The origin of the side-emitted light is Rayleigh scattering, from defects generated in the core during the ultraviolet inscription of the grating. Because the intensity of the scattered light at axial position z is proportional to the light intensity inside the core at z, a spatially resolved measurement of the

scattered light clearly shows the structure of the resonant light field in the grating. The beauty of the method is that it can be performed over a range of wavelengths. A three dimensional plot of scattered intensity as a function of

spatial position and wavelength shows such grating features as chirp [32], phase discontinuity and apodisation [33].

3.4 Polarisation Properties

The photoinduced index change used to produce a fibre grating is generally anisotropic. This has a

significant impact on the properties of the grating, and can affect the performance of devices which employ the

gratings. Examples include polarimetric fibre sensors and FBG dispersion compensators [4, 12-16], where the phase

of the reflected light is of critical importance. Measurements of UV induced birefringence using transmission

146
ellipsometry have been reported [34, 35]. These papers identified polarisation of the UV Ught and geometrical

asymmetry of the writing process as the origins of the anisotropy. Stepanov et al [36] have developed a simple and

inexpensive reflection ellipsometry technique to characterise the polarisation properties of FBGs. Reflection

ellipsometry is a technique based on measurements of the states of polarisation of the incident and reflected waves. The essential optical components for the measurement are a polarising beamsplitter and a quarter wave plate. As

mentioned in section 3.2, Ouellette and Stepanov [28] used essentially the same measurement setup to show that the retardance spectrum of a chirped FBG can be translated into a group velocity delay spectrum.

4. CONCLUSIONS

Fibre gratings have found many applications in WDM telecommunication systems. It is likely that commercial WDM systems will employ fibre gratings, in fihering, in dispersion compensation, in sources and

amplifiers and for multiplexing and demultiplexing. Accurate, fast and convenient measurements of the optical response and structure of fibre gratings have been seen to be crucial in designing gratings to suit particular

applications, and in evaluating the performance of the fabricated gratings. Several such techniques have been

presented.

REFERENCES

[I] G. Meltz, W.W. Morey and W.H. Glenn, Formation of Bragg Gratings in Optical Fibers by Transverse Holographic Method, Optics Lett., 14, 823-5 (1989)

[2] A.M. Vengsarkar, P.J. Lemaire, J.B. Judkins, V. Bhatia, T. Erdogan and J.E. Sipe, Long-Period Fiber

Gratings as Band-Rejection Filters, J. Lightwave Tech., 14, 58-65 (1996)

[3] B.J. Eggleton, P. A. Krug, L. Poladian and F. Ouellette, Long Periodic Superstructure Gratings in Optical

Fibres, Electron. Lett., 30, 1620-2 (1994)

[4] P. A. Krug, T. Stephens, G. Yoffe, F. Ouellette, P. Hill and G. Dhosi, Dispersion Compensation Over

270km at lOGbit/s Using an Offset-Core Chirped Fibre Bragg Grating, Electron. Lett., 31, 1091-2 (1995)

[5] D. B. Stegall and T. Erdogan, Long-Period Fiber Grating Devices Based on Leaky Cladding Mode

Coupling, Bragg Gratings, Photosensitivity, and Poling in Glass Fibers and Waveguides: Applications and

Fundamentals, OFC 1997 Technical Digest Series, vol. 17, paper BSuB2, pp. 16-8 (1997)

[6] T.J. Morgan, R.S. Tucker and G. Yoffe, Passive optical Equalization of Wavelength Converters Based on Four-Wave Mixing in Semiconductor Optical Amplifiers, Photonics Tech. Lett, 10, 522-4 (1998)

[7] P.F. Wysocki, J. Judkins, R. Espindola, M. Andrejco, A. Vengsarkar and K. Walker, Erbium-Doped Fiber

Amplifier Flattened Beyond 40nm Using Long-Period Grating, OFC'97 Proceedings, postdeadline paper PD2 (1997)

[8] H.S. Kim, S.H. Yun, l.K. Kwang and B.Y. Kim, All-Fiber Acousto-Optic Tuneable Notch Filter with

Electronically Controllable Spectral Profile, Optics Lett, 22, 1476-8 (1997)

[9] V. Mizrahi, T. Erdogan, D.J. DiGiovanni, P.J. Lemaire, W.M. MacDonald, S.G. Kosinski, S. Cabot and

J.E. Sipe, Four Channel Fiber Grating Demultiplexer, Electron. Lett., 30, 780-1 (1994)

[10] C.R. Giles and V. Mizrahi, Low-Loss Add/Drop Multiplexers for WDM Lightwave Networks, Proc. iOOC'95, paper ThC2 (1995)

[II] L. Quetei, L. Rivoallan, E. Delvaque, H. Poignant, M. Monerie and T. Georges, Programmable Fiber

Grating Based Wavelength Demultiplexer, OFC'96 Proceedings, paper WF6 (1996)

[12] B.J. Eggleton, P. A. Krug, L. Poladian, K.A. Ahmed and H.-F. Liu, Experimental Demonstration of

Compression of Dispersed Optical Pulses by Reflection from Self-Chirped Optical Fibre Bragg Gratings,

Optics Lett., 19, 877-9, (1994)

[13] F. Ouellette, P.A. Krug, T. Stephens, G. Dhosi and B. Eggleton, Broadband and WDM Dispersion

Compensation Using Chirped Sampled Fibre Bragg Gratings, Electron. Lett., 31 (11), 899-901 (1995)

[14] T. Stephens, J. Arkwright, K. Hinton, D.Y. Stepanov, P.A. Krug, B. Smith, G. Dhosi, G. Yoffe and F. Ouellette, Fiber Gratings for Dispersion Compensation, Invited paper at OFC'98, paper TuMl, p. 71 (1998)

[15] R. Kashyap, A. Ellis, D. Malyon, H.-G. Froehlich, A. Swanton and D.J. Armes, Eight Wavelength x lOGb/s

Simultaneous Dispersion Compensation Over 100km Single-Mode Fiber Using a Single 10 Nanometer

147
Bandwidth, 1.3 Meter Long, super-Step-Chirped Fiber Bragg Grating With a Continuous Delay of 13.5 Nanoseconds, Proceedings of ECOC'96, paper ThB.3.2 (1996)

[16] B. Eggleton, T. Stephens, P.A. Krug, G. Dhosi Z. Brodzeli and F. Ouellette, Dispersion Compensation

Using a Fibre Grating in Transmission, Electron. Lett., 32, 1610-1 (1996)

[17] J.-L. Archambault and S.G. Grubb, Fiber Gratings in Lasers and Amplifiers, J. Lightwave Tech., 15, 1378-

90(1997)

[18] P.A. Morton, V. Mizrahi, P.A. Andrekson, T. Tanbun-Ek, R.A. Logan, P. Lemaire, D.L. Coblentz, A.M.

Sergent, K.W. Wecht and P.F. Sciortino Jr., Mode-locked Hybrid Soliton Pulse Source with Extremely

Wide Operating Frequency Range, Photonics Tech. Lett., 5, 28-31 (1993)

[19] J. Chow, G. Town, B. Eggleton, M. Ibsen, K. Sugden and 1. Bennion, Multiwavelength Generation in an Erbium-Doped Fiber Laser Using In-Fiber Comb Filters, Photonics Tech. Lett., 8, 60-2 (1996)

[20] G.E. Town, K. Sugden, J. Williams, I. Bennion and S.B. Poole, Wide Band Fabry-Perot-Like Filters in Optical Fibers, Photonics Tech. Lett., 7, 78-80 ( 1 995)

[21] M. Ibsen, B.J. Eggleton, M.G. Sceats and F. Ouellette, Broadly Tuneable DBR Fibre Laser Using Sampled

Fibre Bragg Gratings, Electron. Lett., 31, 37-8 (1995)

[22] S.G. Grubb et al. High-power 1.48jum Cascaded Raman Laser in Germanosilicate Fibers, Proceedings of

Optical Fiber Amplifiers and Their Applications'95, paper SaA4 (1995)

[23] S.G. Grubb, T. Erdogan, V. Mizrahi, T. Strasser, W.Y. Cheung, W.A. Reed, P.J. Lemaire, A.E. Miller, S.G.

Kosinski, G. Nykolak and P.C. Becker, 1.3/jm Cascaded Raman Amplifier in germanosilicate Fibers,

Proceedings of Optical Fiber Amplifiers and Their Applications'94, paper PD3 (1994)

[24] L. Poladian, Group Velocity Reconstruction for Fibre Bragg Gratings in Reflection and Transmission,

Optics Lett., 22, 1571-3 (1997)

[25] J. Canning and M.G. Sceats, n-Phase-Shifted Periodic Distributed Structures in Optical Fibres by UV Post- Processing, Electron. Lett., 30, 1344-5 (1994)

[26] D. Yu. Stepanov, J. Canning and Z. Brodzeli, Real-Time Characterisation of Fibre Grating Fabry-Perots

and Phase-Shifted Gratings with lOOkHz Resolution, submitted to Symposium on Optical Fiber

Measurements (1998)

[27] S. Ryu, Y. Horiuchi and K. Mochizuki, Novel Chromatic Dispersion Measurement Method Over

Continuous Gigahertz Tuning Range, J. Lightwave Techno!., 7, 1 177-80 (89)

[28] F. Ouellette and D. Yu. Stepanov, A New Technique for Measuring the Group Delay of Chirped Fiber

Bragg Gratings, OFC'97 Tech. Digest, paper WJl, pp. 153-154 (1997)

[29] P.A. Krug, R. Stolte and R. Ulrich, Measurement of Index Modulation Along an Optical Fiber Bragg

Grating, Optics Lett., 20, 1767-9 (1995)

[30] F. Ouellette, P.A. Krug and R. Pasman, Characterisation of Long Phase Masks for Writing Fibre Bragg

Gratings, Optical Fibre Tech., 2, 281-284 (1996)

[31] J. Canning, M. Janos and M.G. Sceats, Rayleigh Longitudinal Profiling of Optical Resonances Within

Waveguide Grating Structures Using Sidescattered Light, Optics Lett., 21, 609-1 1 (1996)

[32] J. Canning, M. Janos, D. Yu. Stepanov and M.G. Sceats, Direct Measurement of Grating Chirp Using

Resonant Side Scatter Spectra, Electron. Lett., 32, 1 608- 10(1 996)

[33] J. Canning, D.C. Psaila, M.G. Sceats and M. Janos, Fibre Grating Characterisation Using Side-Scatter

Techniques, Proc. Symposium on Optical Fiber Measurements'97, pp. 6-9 (1997)

[34] T. Erdogan and V. Mizrahi, Characterization of UV-Induced Birefringence in Photosensitive Ge-Doped

Silica Optical Fibers, J. Optics Soc. Am. B, 11, 2100-5 (1994)

[35] A.M. Vengsarkar, Q. Zhong, D. Inniss, W.A. Reed, P.J. Lemaire and S.G. Kosinski, Birefringence

Reduction in Side-Written Photoinduced Fiber Devices by a Dual-Exposure Method, Optics Lett., 19,

1260-2 (1994)

[36] D. Y. Stepanov, I.M. Bassett, T. Ryan and F. Ouellette, Ellipsometry of Fibre Bragg Gratings, Proc. Australian Conf on Optical Fibre Technology, pp. 164-7 (1995)

148
Real-time characterisation of fibre grating Fabry-Perots

and phase-shifted gratings with 100 kHz resolution

D. Yu. Stepanov, J. Canning, and Z. Brodzeli

Australian Photonics Cooperative Research Centre

Optical Fibre Technology^Centre, The University of Sydney

Abstract

We demonstrate a real-time measurement technique capable of sub-MHz resolution for characterization of fibre Bragg gratings in transmission.

1. Introduction

Phase-shifted fibre Bragg gratings (FBG) [1] and distributed Bragg reflector (DBR) Fabry-Perot

(FP) interferometers [2] have applications in dense wavelength-division multiplexing (WDM) systems, distnbuted-feedback (DFB) fibre lasers, and photonic signal processing. A Tr-phaseshift

introduced in the physical centre of a uniform fibre Bragg grating results in a transmission notch in

the middle of the grating bandgap. Depending on the grating strength or the interferometer finesse,

the bandwidth of the notches may be very narrow, often well below the 1 pm (-125 MHz at 1550

nm) resolution of semiconductor tunable laser sources commonly used for spectral characterization of fibre Bragg gratings. Slow environmental perturbations may affect the results of the conventional measurements when the laser source is scanned across the wavelength range of interest.

When setting each wavelength, the laser experiences relaxation oscillations and it takes hundreds

of milliseconds to stabilize the lasing frequency. Further, longer delays are necessary if absolute wavelength measurements, obtained by continuous referencing to a slow scanning Michelson

interferometric wavemetre, are desired.

In this paper, we demonstrate a simple technique for real-time measurements of spectral features

of fibre Bragg gratings on a sub-MHz scale.

2. Measurement technique

The schematic of the experimental setup is shown in Figure 1 and consists of a tunable laser source (TLS), a network analyzer (NA), an amplitude electro-optic modulator (EOM), a photodetector

(PD), an oscilloscope and a grating device under test. The NA serves as a source of radio-frequency

(RF) signal, with the frequency swept across the band of interest. The RF signal drives the EOM

and the frequency separation between the optical carrier and the modulation sidebands is swept

over time. By blocking the carrier and one of the sidebands, the remaining sideband acts as a high-resolution light source which is swept across the RF band.

149
Tunable

Laser Source

Electro-optic Modulator m Av(r) Vq±Av

Network

Analyser

DBR-FP

Photo-diode

Vn-Av

Oscilloscope

Figure 1. Schematic of the high-resolution

measurement setup.

1540,4 1540,6 1540,8 1541,0 1541,2 Wavelength, nm

Figure 2. Grating transmission spectrum

measured (open circles) using conventional technique and simulated (solid line)

using the parameters shown in the text.

3. Experimental results and discussion

In this work, we characterized high-finesse DBR-FP interferometers and phase-shifted FBGs. An

optical spectrum of a DBR-FP measured in transmission using a conventional setup which consists

of a TLS with 0.001 nm resolution and a PD is shown in Figure 2 with open circles. The trans- mission notch in the bandgap is hardly resolvable using this setup. We also calculated the grating

transmission using the following parameters: k =8.9 cm~\ L =11.17 mm, =1540.867 nm,

Q =-0.065 nm/cm^, where k is the grating strength, L is the total length of the DBR-FP consisting of two gratings 5.85 mm and 3.95 mm long and a 1.2 mm gap between them, is the Bragg

wavelength of the grating, Q is the quadratic chirp. Some deviation of the measured spectrum from

the calculated one can be explained both by the deviation of the real DBR-FP from our model and

by systematic experimental errors introduced into the measurement setup by the TLS. The laser was found to produce mode hops within 80 pm range when step-tuned either because of the TLS

software faults or because of the feedback from the fibre grating. The -25 dB noise floor is determined by the spontaneous emission outside the lasing wavelength. The spontaneous emission

noise also contributes to the measurement results at the edges of the grating bandgap causing the

'rounding' effect. In the high-resolution measurements, the TLS wavelength was tuned close to the transmission

notch. The grafing reflectors used in the interferometer were suflhciently broad so that on one side

of the carrier frequency one sideband is reflected whilst on the other side the other sideband is swept across the transmission notch as the modulation frequency is scanned.

The transmitted signal is plotted against the sweep time (versus modulation frequency as the

modulation frequency was swept) on the oscilloscope screen. The whole RF span required to cover

the transmission notch could be processed within tens of milliseconds. The frequency resolution was limited by the accuracy of the RF applied to the modulator, the TLS linewidth (-100 kHz) and

the response time of the photodiode. In this experiment, the TLS linewidth is the major limiting

factor. The high-resolution spectrum of the DBR-FP is shown in Figure 3. The horizontal scale

defines the frequency downshift, Aiy, of the red sideband from the carrier frequency which was set

to uq « 194.446 THz (1541.777 nm). Hence, the absolute frequency of a spectral feature can be

determined ?ls u = uq — Au. This could be important for characterization of an environmentally

150
stable packaged device in an application where an accurate measurement of the absolute frequency

is required, such as referencing of the optical frequency scale for WDM communications [3]. The

device was not packaged. Moreover, there was a different tension applied to the device for the

conventional and high-resolution measurements explaining the mismatch of the wavelength scale

in Figure 2 with uq.

J

155.6 MHz

<

22.2 MHz > < u

200 400 500 800 1000

Av, MHz

1200

Figure 3. Polarization splitting of the

transmission passband observed using

setup shown in Figure 1.

22.2 MHz

0

500 600 700 800 Av , MHz

900

Figure 4. Transmission passband measured (open circles) for a single polarization using a linear polarizer and calculated with 1.25 MHz (solid line) and 125

MHz (open squares) step using calculation parameters shown in the text.

Polarization splitting of 155.6 MHz is apparent from the high-resolution spectrum shown in

Figure 3. The laser source used in the experiment had a single linear polarization output which

evolved by the time it reached the structure, allowing both polarization states of the device to be

observed. Birefringence of the fibre sample calculated from the polarization spUtting is about 10~^

which is the same order of value as measured in [4] for the fibre without a grating and fabricated

using the same recipe.

The optical spectrum shown in Figure 4 with open circles was obtained by fihering the output

using a linear polarizer aligned parallel to one of the DBR-FP birefringent axes. The bandwidth of

the transmission notch was found from the high-resolution spectra to be 22.2 MHz implying very

high finesse of the DBR-FP. A frequency mismatch of about 100 MHz between the subsequent

measurements of the transmission passband represented in Figures 3 and 4 probably arose from

environmental perturbations during the insertion of the polarizer into the setup since the grating was not properly packaged.

The optical spectrum calculated using the same parameters that we used to fit the transmission

spectrum in Figure 2 is represented in Figure 4 with a solid line. It shows excellent agreement with

the experimentally measured spectrum. For comparison, the transmission passband calculated

with the minimum resolvable step of the TLS of 0.001 nm is shown in Figure 4 with open squares.

Clearly, the conventional technique fails to resolve the passband as it is represented by only one

data point in the plot. In the experiment, a modulation sideband of the modulated optical carrier was used and its transmission through grating devices was measured while sweeping the modulation frequency.

Another approach, similar to that described in [5], involves measuring the beat frequency between

the carrier and the modulation sideband. A single sideband (SSB) modulation technique should be

151
employed in this case. The technique in its current implementation does not allow the measurement of absolute values

of transmission through the notch. This requires a calibration to the signal passed outside the

bandgap. Since outside the bandgap the sideband used in the measurements could not be separated

from the carrier and the other sideband, such a calibration has not been performed. The aim of the

measurements was to spectrally resolve the passband and measure its bandwidth.

4. Conclusion

In conclusion, we have demonstrated a simple and accurate real-time high-resolution technique for

measuring spectral features in fibre Bragg grating transmission spectra. The resolution was limited

to the 100 kHz linewidth of the semiconductor laser source used in the measurement setup. The

technique can be used both for precise characterization of fine structure observed in low resolution

optical spectra of fibre grating devices, and in sensor applications where environmental variables

are measured versus a change in grating spectral features.

5. Acknowledgements

The authors would like to acknowledge G. Yoffe and B. Smith from the Australian Photonics CRC

for fabrication of the grating structures.

6. References

[1] J. Canning and M. G. Sceats, "7r-phase-shifted periodic distributed structures in germanosilicate fibre by uv post-processing", Electron. Lett., 30(16), 1344-1345, (1994).

[2] W. W. Morey, T. J. Bailey, W. H. Glenn, and G. Meltz, "Fiber Fabry-Perot interferometer

using side exposed fiber Bragg gratings", in Tech. Digest of Conf. on Opt. Fiber Commun.

(OFC'92), 1992, paper WA2.

[3] C. Gamache, M. Tetu, C. Latrasse, N. Cyr, M. A. Duguay, B Villeneuve, "An optical fre- quency scale in exact muldple of 100 GHz for standardization of mulfifrequency communications", IEEE Photon. Technol. Lett., 8(2), 290-292 (1996).

[4] F. Ouellette and D. Yu. Stepanov, "A new technique for measuring the group delay of chirped

fibre Bragg gratings", in Tech. Digest of Conf. on Opt. Fiber Commun. (OFC'97), Dallas,

Texas, February 16-21, 1997, paper WJl.

[5] J. E. Romn, M. Y. Frankel, and R. D. Esman, "High resolution technique for characterizing

chirped fiber gratings", in Tech. Digest of Conf. on Opt. Fiber Commun. (OFC'98), San Jose,

California, February 11-11, 1998, paper TuA5.

152
OTDR-like dispersion-map measurements

Jiirgen Gripp and Linn F. Mollenauer

Bell Labs, Lucent Technologies, Holmdel, New Jersey 07733

We have developed an instrument for fast, convenient and accurate measurement of the

dispersion map D{z) of optical fibers. Access to only one end of the fiber is required, making

this technique perfectly suited for diagnosis of installed as well as spooled fibers. While

low-loss fibers up to 45 km can be measured directly, we have also demonstrated a extended

range of 75 km in low-loss fiber, and 22.5 km in high-loss, dispersion-compensating fiber by

using Raman gain.

Since dispersion management has become the key in the design of ultra high capacity fiber

optic transmission systems, there is a great need for fast, precise and convenient measurement

of dispersion maps D{z). Various techniques, based on modulational instability \ or the phasematching of four-wave-mixing products'^'^ have been proposed and tested. However, all these

approaches suffer from certain common drawbacks, viz., they measure the wavelength of zero

dispersion Aq instead of D{z), they require time consuming data collection over a wide wavelength

range, and the resulting amplitude profile does not necessarily allow an unambiguous reconstruction

of spatial information. Our instrument, on the other hand, overcomes all of these disadvantages by

using an optical-time-domain-reflectometry (OTDR-like) technique previously invented by one of

US'* and subsequently adopted by several other groups^'^. A measurement typically takes no more

than a few seconds, and the data is unambiguously and almost instantaneously analyzed. We have

successfully measured samples of every important fiber type, including various dispersion-shifted

fibers with 0 < |D| < 11 ps/nm/km, standard fiber with D ^ 17 ps/nm/km, and dispersioncompensating fiber with D ^ —100 ps/nm/km.

At the basis of our method are the four- wave mixing processes

(see Fig.l). Two photons at uji (or UJ2) combine to create one photon at uj2 (or ci;i) and one

Stokes (or anti-Stokes)-photon. The four frequencies are equally spaced by 5u> to satisfy energy

conservation. For non-zero dispersion these two processes are subject to wave-vector mismatches

that axe directly proportional to D(cJi) and D{u)2)- For example process (la) leads to

UJs = 2uJ\ — UJ2 , (la)

(lb)

Sk{uji) =k2 + ks - 2ki = -2ncD{uji) 2ncD[X,) (^) (2)

153
Fig. 1. Four-wave mixing spectrum

(For process (lb), replace the subscript 1 with 2 and the subscript A with S.) It is important to

notice that this result is independent of third order dispersion"^. Our instrument initiates measure- ment of this wave-vector mismatch by launching two strong submicrosecond pulses at Ai and A2

simultaneously into the fiber (see Fig. 2). While the pulses travel down the fiber, the four-wave

mixing fields they create (at A5 and A^) oscillate in intensity with a spatial frequency that is

directly proportional to the wave-vector mismatch. For the Stokes field, for example, the spatial

frequency Fs is given by

\Sk{X,]

27r cZ^(Ai)(y)' (3)

The instrument measures the spatial oscillation, either at the Stokes or the anti-Stokes frequency,

as temporal oscillations in the received, Rayleigh-backscattered light. The backscattered light at

either A5 or A^ passes a narrow filter, is detected, and the resultant electrical signal of frequency

f{t) = ^^Fsiz) is digitized. The signal is then converted into the dispersion map D{z) through an

efficient, FFT-based algorithm (described below).

To achieve the highest possible accuracy and spatial resolution, several properties of the

launched pulses have to be carefully controlled: First, the power levels have to be kept below

the level for stimulated Raman scattering, and, if Aq < Ai 2, below that for modulational instability. Second, the linewidths have to be large enough to avoid stimulated Brillouin scattering.

SWITCH

' MFL

_ _ _ _ _ _ _ I

PC

WITH

ADC

FILTER

50-500kHz

CIRCULATOR

EDFA

AMPLIFIER

TUNABLE

ETALON

<1-

^3

OR

DETECTOR

FIBER

UNDER

TEST

Fig. 2. Schematic of the instrument. A multi-frequency laser (MFL)^ generates a

few milliwatts CW at Ai and A2 and shapes 0.8 /xs pulses through use of an extra-cavity

semiconductor optical amplifier switch. The pulses are then amplified up to IW and sent

into the fiber under test. A circulator directs the Rayleigh-backscattered light into another

amplifier, through a narrow filter (at either A5 or A^) and into a detector.

154
0 5 10 15 20 25

2 (km)

Fig. 3. a: Signal of a 25 km span of dispersion shifted fiber, b: Solid thick curve:

Dispersion at A = 1552 nm. Solid thin curve: D measured from left to right. Dashed thin

curve: D measured from right to left.

Finally, 5\ must be choosen according to the approximate value of Z), to yield the best compromise between spatial resolution, which increases with increasing 5\, and signal strength, which

decreases with increasing 5\. Also, of course, 5X must be stable and precisely defined, since the

measured dispersion is inversely proportional to (see Eq. (3)). Table 1 shows values typically

used by the instrument.

Figure 3a shows a typical signal returned from a 25 km span of dispersion shifted fiber, taken at 1555 nm with 6X = 3.6 nm. To compute the curves of implied dispersion (Fig. 36), the instrument

takes a FFT of the signal, eliminates the negative part of the resulting symmetric spectrum, and

then performs an inverse FFT; this effectively converts the linear, real-valued oscillation of Fig. 3a

into a circular oscillation in the complex plane. The curvature of the circular oscillation is directly

proportional to D{z) and the dispersion is obtained by taking its integral. It should be noted that

the algorithm is fast and accurate, requiring less than one second on a 133 MHz Pentium processor.

Fiber Type |L>(1550nm)| (ps/nm/km) SX (nm) spatial resolution (m)

dispersion compensating 80 to 100 0.6 220 to 280

standard ^17 1.2 ^330

dispersion shifted 1 to 10 1.8 to 3.6 250 to 600

Table 1. Parameters used for difTerent fiber types.

155
Fig. 4. Possible evolution of the Stokes vector, a: without coherent field, b: with

coherent field (dashed arrow). Case a leads to no measurable power change on the Icist turn,

in case b the oscillations can be detected all the way through.

To illustrate the precision of the instrument, we plot D for two measurements, taken from either

end of the span. The measurements have a spatial resolution of 500 m and a relative uncertainty

in D of less than 2%.

We have recently made two other significant improvements. The first consists of adding a small

coherent "seed" field at the signal frequency A5(A^). We generate this seed by sending the two

probe fields at Ai and A2 through a semiconductor optical amplifier. The four-wave mixing fields

it produces are small compared to the probe fields, but are considerably larger than the four-wave

mixing fields generated in the fiber itself. The resulting displacement of the net signal field away

from the origin (see Fig. 4) has two benefits: First, the seed enhances the intensity of the measured

signal, since the net signal now represents the beat between the (large) seed and the (smaller) fiber

signal. Second, a potential problem depicted in Fig. 4 is avoided, i.e., that a sudden change in the

dispersion could cause the Stokes field to rotate around the origin, leading to a much decreased

intensity oscillation. That is, with the coherent seed field present, the oscillation of the field vector

always translates into an intensity oscillation.

The second improvement extends the range of the instrument with the help of Raman gain.

The signal decay rate is four times higher than the absorption rate of the fiber under test"^. Given

the current sensitivity of the instrument, this limits the range of regular measurements to under

50km, even for fibers with lowest loss coefficients. However, we can maintain a measurable signal

power over significantly longer distances by sending a few hundred milliwatts of 1450 nm Raman

pump light into the fiber both from the near end and the far end (see Fig. 5). Note that even

Dispersion Map

Instrument, see Figure 2 WDM

FIBER

UNDER

TEST

Cascaded Raman Laser 1

Cascaded Raman Laser 2

Fig. 5. To extend the range of the instrument, two cascaded Raman lasers provide

Raman gain from both ends of the fiber.

156
in this case, the instrument itself requires access from only one end of the fiber, since the pump

lasers do not require any synchronization with the measurement. Thus, even with Raman gain, the

OTDR-like technique maintains its unique and very important ability to measure the dispersion

maps of installed fibers in the field. Figure 6 shows a measurement of 75 km of an experimental dispersion shifted fiber (DSF)

with a loss coeflficient of 0.21 dB/km. First we performed the measurement on the entire span,

pumping from both ends, and then on sub-spans of 25 km length each, without pump. Note that

without pumping, a measiurement of the entire 75 km would not be possible, since in that case the signal power would decrease by an untenable 63 dB over the length of the span. From the

nearly perfect fit of the two resultant sets of curves, we infer that the measurement uncertainty is

small, i.e., the accumulated dispersion deviates by less than ±0.1 ps/nm over the first 25 km, by

less than ±0.2 over the next 25 km, and by less than ±0.3 km over the last 25 km. This means

that the accumulated dispersion has an exceedingly small relative uncertainty of ±0.3% over these

distances. Finally, note that the dispersion parameter D varies from about -2 to -0.5 ps/nm/km.

This variation shows how important it is for the construction of dispersion managed systems to be

able to measure the actual dispersion maps of the fibers.

Figure 7 shows a measurement of 22.5 km of dispersion compensating fiber (DCF) with a loss

coefficient of 0.51 dB/km, pumped only from the far end. We measured the fiber from both ends and

obtained equally small uncertainties as in the case of the dispersion shifted fiber. The measurements

of these two fiber types with very different dispersion values demonstrate the flexibility of the

-120 -2.5

E

cM

c

_o

'5

0)Q.

(A

z(km)

Fig. 6. Dispersion shifted fiber, measured with pump light from both ends. 5\ = 2.4 nm. Solid thick curve: Dispersion at A = 1552 nm obtained from one measurement of the

entire span. Dashed thick curve: Dispersion obtained from three separate measurements of

25 km each. We shifted the dashed curve up by 10 ps/nm for better visibility of the two

virtually identical curves. Solid thin curve: D from the measurement of the entire span. Dashed thin curve: D from the three separate measurements. The two vertical dotted lines

indicate the points where the fiber was cut.

157
z (km)

Fig. 7. Dispersion compensating fiber (DCF), mecLSured with counterpropagating

pump light. SX — 0.6 nm. b: Thick curve: Dispersion at A = 1554 nm. Thin curve:

Dispersion parameter D.

OTDR-like dispersion measurement technique. In the near future, this instrument is most likely

going to become an indispensable tool both for the design of dispersion managed fiber transmission

systems and for the measurement of installed fibers.

We are very grateful to Christopher Doerr, Charles Joyner, and Larry Stulz for supplying us with multi-frequency lasers and to Olivier Ondet for helpful discussions.

References

1. S. Nishi and M. Saruwatari, Electron. Lett. 32, 579 (1996).

2. R. M. Jopson et a/., Electron. Lett. 31, 2115 (1995).

3. I. Brener, P. P. Mitra, and D. J. Thompson, in Optical Fiber Communication Conference,

Vol. 2 of OSA Technical Digest Series (Optical Society of America, Washington, D. C, 1998),

pp. 344-345.

4. L. F. Mollenauer, P. V. Mamyshev, and M. J. Neubelt, Opt. Lett. 21, 1724 (1996).

5. N. G. R. Broderick, D. J. Richardson, and L. Dong, IEEE Photon. Technol. Lett. 9, 1511

(1997).

6. M. Artiglia et a/., in Optical Fiber Communication Conference, Vol. 2 of OSA Technical Digest

Series (Optical Society of America, Washington, D. C, 1998), pp. 179-180.

7. C. R. Doerr, C. H. Joyner, L. W. Stulz, and J. C. Centanni, IEEE Photon. Technol. Lett. 9,

1430 (1997).

158

J.
MEASUREMENT OF THE SPATIAL DISTRIBUTION OF THE

ZERO-DISPERSION WAVELENGTH IN DS FIBERS

BY BIDIRECTIONAL LINEAR REFLECTOMETRY

A. Rossaro M. Schian'o, M. Dainese*, F. Lovisolo*

CSELT, via Reiss Romoli 274, 10148 Torino (Italy)

*Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino (Italy)

Abstract

A linear bidirectional OTDR technique for the nneasurement of the spatial distribution of the zero

dispersion wavelength has been applied to dispersion shifted terrestrial plants. In this paper results are presented and compared with other measurement methods demonstrating the good accuracy and

reliability of the reflectometric technique.

1. Introduction

Measurements of Chromatic Dispersion (CD) uniformity are assuming high importance not only in the

field of fiber optic production quality assurance, but also in the design of high capacity Wavelength

Division Multiplexed (WDM) telecommunication systems. In fact, in amplified WDM transmission systems,

where many high power channels are transmitted on a relatively narrow spectral range, impairments may

be caused by Four-Wave Mixing (FWM) when the link CD is very low, i.e. in systems operating on

Dispersion Shifted (DS) fibers [1]. While new plants can be designed with a careful choice of CD for each

cable section, or even with dispersion-managing techniques, problems could originate in the upgrade of existing DS plants.

In this case, since FWM efficiency depends dramatically on the CD spatial distribution, it would be useful

to measure such distribution by means of a simple, field usable technique. A large amount of experimental work has been carried out on this topic and many laboratory results have been reported in literature [2, 3, 4]. Also in the standardization bodies there is a strong interest on chromatic dispersion

uniformity. Some contributions have already been presented, showing that a high uniformity of the Zero

Dispersion Wavelength (ZDW or "Kq) is required for Non-Zero Dispersion (NZD) fibers and vice-versa a

high non-uniformity is desirable for DS fibers [5]. This paper reports the results of ZDW measurements performed on a DS fiber installed plant by means of the bidirectional reflectometry technique in linear regime. This technique has been chosen for the

simplicity of the measurement set-up in comparison with non-linear reflectometry, and has been

especially tailored for measurements on DS fibers. The bidirectional linear reflectometry has been

demonstrated enough accurate to perform a complete spatial characterization of a fiber in terms of Mode

Field Diameter (MFD), CD and ZDW.

Results indicate that, at least in the considered plant, the concatenation of different fibers leads to large

fluctuations of ZDW along the fiber length.

2. Bi-directional OTDR technique

In the following the method for measuring ZDW distribution based on bi-directional OTDR technique is summarized, with particular emphasis on its application to DS fibers.

159
It is well known that a bi-directional OTDR technique [6] allows to measure the spatial distribution of waveguide parameters [7]: in particular Mode Field Diameter (MFD) can be precisely measured in function of distance, provided that a reference fiber with a known MFD is available. Starting from the

ja{x)dx

expression of the backscattered optical power: P(z) = Poa^(z)S(z)-\0 °

, it is straightforward to calculate the half-sum of the bidirectional OTDR traces:

where as is the backscattering coefficient, S(z) is the recapture factor, and A is a constant. For step index

fibers S(z) can be expressed in function of MFD d(z) (Petermann II definition):

2

f-> n

S{z) = - 6/ i n, - ^2

diz)

\ a )

If a reference fiber is inserted before the fiber under test with a known MFD at location Zq, and neglecting

longitudinal variations of as , MFD in function of distance can be easily calculated:

I\z) = I(z) - I{Zo) = 20Log,, o d{z,X) = d{z„X)lO~^

Although previous relations hold strictly for step index fibers, their validity extends to DS fibers with

negligible errors [8]. Examining now CD, it can be considered as the sum of different contributions:

where: = is the material dispersion, D^{z,X)= ^ ^J A ddis the

c dX^ '' ' K\n,-d\z)\.~ 2-d{z) dX

waveguide dispersion and the D^^ is the profile dispersion whose contribution has been considered

negligible (in a first approximation).

It can be noted that waveguide contribution to chromatic dispersion can be easily expressed in terms of a wavelength derivative of MFD. From this point the analysis of Step Index (SI) and DS fibers will be

performed separately. For SI fibers an analytical expression of the wavelength dependence of MFD can

be derived, while for DS fibers, since the application of approximation methods, such as Equivalent Step

Index (ESI), does not produce good results, the following numerical approach has been used. MFD

behavior in function of wavelength, at a given position z, was considered linear between measured values

at 1310 and 1550 nm and the corresponding wavelength derivative was used for the calculation of the

local waveguide dispersion. This technique named Local Linear Interpolation, leads to accurate results as

is shown in the next paragraph.

Material contribution to chromatic dispersion is calculated by means of the well-known Sellmeier equation.

Finally, assuming a linear dependence of total chromatic dispersion from wavelength, it is easy to calculate the spatial distribution of Xq from the measured spatial distribution of CD evaluated at two or more wavelengths.

3. Measurement results

In this paragraph two examples of the evaluation of MFD and zero dispersion wavelength are reported. In the first case measurements have been performed in the laboratory on a concatenation of DS fibers bobbins (2 fibers 1 km long, and 1 fiber 10 km long), accurately chosen for their good longitudinal

uniformity. In the second case, measurements have been performed on a plant, 46 km long, installed in urban ducts. Some important features of the DS fiber plant and of the measurement method as well are

highlighted.

First of all, it can be noted that the reflectometric trace noise has a negligible influence on the

measurement accuracy. Uncertainties in the order of 0.1 ^im and 0.5 nm for Mode Field Diameter (Fig. 1,

160
3) and Zero Dispersion Wavelength (Fig. 2, 4) respectively can be achieved with commercial OTDR even

on relatively long links.

8 7 8685

8483

82

8 1

8 7978

7 7,

Reference

Fiber

MFD@1550nm

10 15 20 km25 30

ZERO DISPERSION WAVELENGTH

Fig. 1

- Measurement of mode field diameter

uniformity on PS fibers

Fig. 2 - Measurement of zero dispersion

wavelength uniformity on DS fibers

Moreover, slight periodic oscillations of both MFD and ZDW are evident in fig. 1 and fig. 2, and may be

attributed to the drawing process.

Obtained results of MFD and ZDW are in very good agreement with the measurements performed, with

other test methods, by the manufacturer, as reported in the following table: MFD by

manufacturer

(VAFF

technique)

MFD by OTDR

(bidirectional

mesurements)

ZDW {Xo) by

manufacturer

(PHASE SHIFT

technique)

ZDW (^o) by OTDR

(bidirectional

mesurements)

Reference Fiber 8.3 8.28 1556 1555.8

Fiber 1 8.4 8.27 1556 1555.8

Fiber 2 8.4 8.28 1556 1556.0

In the second case, the bidirectional OTDR technique has been used on an installed plant to check if a WDM transmission system could be used.

79

7 8 77

7.6

7.5

7 4

7,3

7.2

7 1

7

MFD @ 1550 nm

n r

10 20 30 km

40 50

1570

1565

1560

1 1555

1550

1545

1540

ZERO DISPERSION WAVELENGTH

- -pi—f. un

s

>•

10 20 30 km

40 50

Fig. 3 - Distribution of mode field diameter @1550

nm on an installed DS fiber cable

Fig. 4 - Distribution of zero dispersion wavelength

on an installed DS fiber cable

161
Also in this case this technique seems to give very good results since the average ZDW evaluated by

bidirectional OTDR (1553.6 nm) is very close to the value measured by a Phase Shift instrument (1553.9

nm).

It is straightforward to observe that the considered plant presents quite large longitudinal fluctuations both

in MFD and ZDW. This is due to the concatenation of relatively short sections of cable with different dispersion characteristics.

Transmission experiments carried out on this plant have demonstrated that this configuration of dispersion distribution allows the transmission of 8X2.5 Gbit/s WDM signals without significant impairment even if one of the wavelengths is close to the average ZDW of the fiber. Finally it is worthwhile to underline that by this simple bidirectional OTDR technique it is possible to characterize fibers not only in terms of attenuation and length, but also in terms of attenuation uniformity,

mode field diameter and chromatic dispersion uniformity, giving a great support to fibers testing,

expecially in the field.

4. Conclusions

The present contribution shows that OTDR technique based on bi-directional measurements at different wavelengths can be successfully used to measure local zero dispersion wavelength in DS optical links. Using this simple technique it is therefore possible a preliminary check on the CD distribution of a plant in order to assure the immunity of WDM systems from FWM impairment. Results indicate that, provided that a careful characterization of the reference fiber is performed, measurement accuracy and resolution are so high to allow the detection of fiber imperfection due to drawing process and CD longitudinal variations on several tens of kilometers of fiber. Further developments of the technique are under study, concerning the accuracy and the measurement

of small CD components such as the profile dispersion.

Acknowledgements

Authors express their gratitude to Dr. D. Cuomo, FOS SpA, for helpful discussions on OTDR tecnique.

References

[1] K. Inoue, Four-wave mixing in an optical fiber in the zero dispersion wavelength region, Journal of Lightwave Technology, Vol. 10, No. 11, 1992, pp. 1553-1561.

[2] M. Ohashi, M. Tateda, Novel Technique for measuring longitudinal chromatic dispersion

distribution in singlemode fibers, Electronics Letters, Vol. 29, N. 15, 1993, pp. 426-428.

[3] K. Nakajima, M. Ohashi and Y. Miyajima, Evaluation of chromatic dispersion distribution in a

single-mode fiber, OFMC '97, pp. 235-240.

[4] M. Artiglia et al., Measurement of the dispersion map of installed G.653 fiber links using four wave mixing, OFC'98, pp. 1 79-1 80.

[5] M. Ohashi, H. Okamura, Comments on chromatic dispersion uniformity, 1997 ITU-T Geneva '97 D.67(WP4/15)

[6] M. Artiglia, A. Chiantore et al., OTDR Trace Analysis for Fiber Characteristics Evaluation: a 'bi- directional' approach, SOFM '94 Proceedings, pp.61 -64

[7] P. Di Vita and U. Rossi, Backscattering measurements in optical fibers: separation of power

decay from imperfection conthbutlon. Electronics Letters, Vol. 15, N. 15, 1979, pp. 467-469

[8] M. Ohashi, K. Nakajima, Comments on an alternative test method for mode field diameter, 1998

ITU-T Geneva '98 D.I 57 (WP4/1 5)

162
A NEW TECHNIQUE FOR ZERO-DISPERSION

WAVELENGTH MAPPING IN SINGLE MODE

FIBER WITH HIGH SPATIAL RESOLUTION

I. Brener (1), D.D. Lee (1), P.P. Mitra (1), D. Philen (2), D.J. Thomson (1)

(1) Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill NJ 07974 (igal@lucent.com)

(2) Lucent Technologies, 2000 Northeast Expy, Norcross, GA 30071-2992

W? present a new noninvasive and simple technique for measuring the spatial variation of the zero-dispersion

wavelength (Xq) in single-mode fibers. When applying this technique to dispersion shifted fibers we can resolve fluctuations in Xq of less than O.OSnm with a potential spatial resolution of better than 100m

Introduction

Several non-destructive techniques have been developed

lately in order to measure the chromatic dispersion (D) and

the zero dispersion wavelength (Xq) variations along the length of the fiber Most of the reported techniques rely on the use of four-wave mixing (FWM) as a probe for fluctuations in D and A^. We present a new and simple

technique for measuring Xoiz), i.e. the variation of Xo along

the fiber length, with great accuracy (<l{)Om).

Theory

Assuming that the phase mismatch between all involved

fields is position dependent, the FWM intensity after propagation in a fiber of length L is given by 6-7.

0

I

<t>(z) = K\X^{yyty (1)

Xj and X2 are the pump and probe wavelengths

respectively, AX^A-i-X^. In our terminology we call pump

the laser wavelength closer to the FW^ peak, so Eq. (1) is valid for the FWM field that satisfies C0FWM=2©i-a)2. If we

keep AX fixed then K becomes a constant of the experiment; q has dimensions of a wavevector. Thus the FWM efficiency as a function of X measured at a constant detuning is governed by the Fourier transform of a phaseonly function e'*'^'* weighed by the absorption e'"^. The goal is to obtain the profile X^iz) from the measured

quantity Ifwh/X) or, given I(q}y find (j)(z}- This is a nonlinear inversion problem and is closely analogous to inverting a Fourier transform. In fact, if the phase of the output electric field could be measured in addition to the

intensity, then the inversion problem would be a straightforward inverse Fourier transform. In general, the

loss of phase information would mean one could no longer

invert the Fourier transform. However, in the special case we are deahng with here (namely the magnitude of the function is known both in real and Fourier space), the problem is known to have a unique inverse as long as ^z)

is analytic and ooO

The existence of a unique inverse to the above problem

does not in general guareintee that we wall be able to perform the inversion in practice, because of measuremental uncertainties which lead to imperfect

knowledge of I(q), as well as possible difficulties in the

inversion procedure. Also, in the above procedure,

polarization effects have been neglected, which may in general be important. We find, in practice, that the inversion is possible, both in numerical simulations as well

as in real fiber measurements in which the actual zero

dispersion profile is determined by direct means. In general, the inversion procedure we use is to parametrize

the function <p(z) in terms of a limited number of parameters, and then to find the values of the parameters

that minimize the mean square error between the observations and the specti-um that would be generated by

the parameti-ized (Piz). Several parametrizations are possible; for example, we have used both piecewise linear and an expansion of the phase in terms of a basis set, such

as Chebychev functions.

In any parametrization, there is the important question of

resolution. However, insight may be obtained ft'om the corresponding linear inverse problem. In this case, the resolution would be given by Ax=2n>ic(Xnjax-^min). where

(^ax-^Tnin)' represents the range over which we can acquire

meaningful data. As an exeimple, for a typical 25km fiber spool and using AX^Snm, (Xniax-^Tnin)=20nm, so Ax=80m.

In shorter spools, measuring with \X = 6nm we obtain (ATnax-^mm)=40nm SO Ax=50m. Longer integration time

allows increasing (Xmax-^min) therefore mapping X({z) with

a potential spatial resolution of less than 50m is feasible.

ECL1

5

5

Power &P0I.

Pol. Monitor

Tracking — > BP Filters I |_kAJ

Pol.

ECL2

OSA

Rber

Lockin

Figure 1: Experimental Setup

Experimental setup used to measure the fluctuation in Xq. The two lasers are scanned at a constant detuning and the FWM peaks are recorded with either an optical spectrum

analyzer (OSA) or a lockin amplifier

163
Experiment and results The experimental setup used in this work is shown in Fig. 1. Two external cavity tunable lasers (ECL) are combined with collinear polarizations and launched into the

fiber under test. We also use tracking bandpass filters at the output of each laser in order to suppress any spurious ASE

background and improve the dynamic range of the measurement. The power of the tunable lasers is kept at a few dbm and thus nonlinearities are not important. The two

tunable lasers are scanned together at a constant detuning

AA and the FWM peaks are analyzed in an optical spectrum

analyzer or using lockin detection. This technique can be

applied to any wavelength range covered by Ao provided

that tunable lasers can be tuned through the wavelength

region of interest. Conventional single mode fiber with

/^o~1.3 |Lim and new dispersion shifted fibers with /^>1,6 |Xm and Ac<l.5 \im can be readily analyzed with our

technique.

——1—

AX=9nm Experiment L=6.7km A ' ' Calculated ] jHiy (b)

10 15 20 25

z (km)

1545 1550 1555 1560 1565 1570 1530 Wavelength (nm)1540 1550 1560 1570 Wavelength (nm)

Figure 2: Example

(a) The calculated Xo(z) distribution for 2 fiber spools of

6.7 and 25.4 km; (b) the experimental FWM tuning curves and the calculated curves using the Xo(z) of Figs. 2(a).

In order to illustrate the capabilities of this technique we

apply our procedure to two dispersion shifted fiber spools

of 6.7 and 25km with average Xq of 1555 and 1550 nm,

respectively. The step curves in Figs. 2(a) correspond to a destructive measurement of Ap for both spools performed

using conventional techniques. The calculated A(j(z) profile

using an improved basis of orthogonalized sines and

cosines is shown by the continuous curve in Figs. 2(a). The

inferred profile matches the observed spread in Ac extremely well. Furthermore, the experimental spread in Ao

is consistent with the width of the FWM tuning curve, as shown in Figs. 2(a). The solid line in Figs. 2(b) show the experimental FWM tuning curves and the dashed curve shows the calculated FWM tuning curve using the inferred

profile of Ao(z) for both fibers. There is a very good

agreement for almost 3 orders of magnitude.

A limitation to this and all previous techniques based on FWM arises from polarization mode dispersion (PMD)

and/or birefringence. This has been largely overlooked and

deserves some discussion. When accounting for the electric

field polarizations, the FWM efficiency in the present

configuration is proportional to

jg^

. ^. If one assumes a

constant birefringence An , this inner product will oscillate

along the fiber length with a period of: 12 A/I A«

For example, if A« - 10"* and AX^lOnm, then Lco/,=12km.

In reality, there will not be a simple expression for this "polarization coherence length" as Am is stochastic. Nevertheless, from this estimate we can expect that the

relative polarization between and £j will not be maintained over long fiber spans if AA, is finite. This will superimpose an unknown spatial oscillation in the FWM

efficiency, which in turn will complicate the inversion process (although it will still be solvable). In order to

illustrate this point, we measured FWM tuning curves for several 25km fiber spools with average PMD in the range

of 0.03-0.2 ps/km!^, and for parallel and perpendicular

polarizations. We find that in the spools with the highest

PMD and for A^8nm the FWM curves for both

polarizations coincide, which means that the relative polarizations are completely lost after a certain length

(probably much shorter than 25km). Consequently, the ultimate resolution that can be achieved by this (and

similar) techniques will be determined by the largest AX. allowed by the PMD of the fiber under study. This holds as long as the inversion is carried out in the way presented in

this paper. One may devise different inversion algorithms

that could accommodate an additional spatial modulation in the FWM efficiency introduced by PMD. Finally, we note

that the procedure presented in this paper can be

generalized in order to map the variations of chromatic

dispersion as a function of position.

In conclusion, we presented a new technique for mapping

the spatial variations in the zero dispersion wavelength

along the fiber length. The technique is easy to implement,

avoids fiber nonhnearities through the use of low power

CW lasers, and has the potential to map spatial fluctuations with an accuracy of better than 100m. The ultimate

resolution is only limited by the polarization mode

dispersion of the fiber.

references

III M. Ohashi and M. Tateda, Electron. Lett. 29, 426

(1993).

Ill Y. Suetsugu, T. Kato, T. Okuno, and M. Nishimura, IEEE Phot. Tech. Lett. 7, 1459 (1995).

/3/ S. Nishi and M. Saruwatari, Electron. Lett. 32, 579

(1996).

lAI M. Eiselt, R.M. Jopson, and R.H. Stolen, J. Lightwave Technol. 15, 135 (1997).

151 L.F. Mollenauer, P.V. Mamyshev, and M.J. Neubelt

Opt. Lett. 21, 1724(1996).

161 K. Hill, D.C. Johnson, B.S. Kawasaki, and R.I. MacDonald, J. Appl. Phys. 49, 5098 (1978).

77/ K. Inoue, J, J. Lightwave Technol. 10, 1553 (1992).

/8/ A.M.J. Huiser, P. van Toorn and H.A. Ferwerda,

Optik 47, 1 (1977).

191 K. Inoue, IEEE J. of Quantum Electr. 28, 883 ( 1992).

164
ANALYSIS OF SECOND ORDER CHROMATIC DISPERSION

VARIATION IN OPTICAL FIBER UNDER LARGE STRETCHING

SIMOHAMED L.M., AUGUSTE J.L., RIOUBLANC J., BLONDY J.M., REYNAUD F.

Stretched fiber suffers opto-geometrical parameters modifications leading to group velocity dispersion

evolution. By means of an interferometric device, ths second order chromatic dispersion is characterized versus fiber stretching. The purpose of this paper is to fit the experimental results with a model taking into account

photo-elastic tensor and wave guide geometry modifications.

Introduction:

In the frame of a large stroke optical fiber delay line implementation using silica fiber ability to induce large and

variable group delay when they are stretched [1,2], we intent to express theoretically the second order chromatic

dispersion evolution versus fiber stretching. The experimental characterization is achieved by using a fiber Mach-Zehnder interferometer. A spectral analysis of the interferometric mixing allows spectral phase

determination and the second order chromatic dispersion is deduced from the phase curvature [3]. Two experimental configurations are performed. The first one consists in the accurate measurement of the second order chromatic dispersion evolution versus stretching. The aim of the second is to calibrate the variable

additional chromatic dispersion induced by the stretching by means of a comparative study using a set of

unstretched fiber samples. In the two configurations the experimental data are plotted versus the generated group

delay.

In this first approach, a theoretical analysis of the experimental results is performed using a standard silica fiber model. Group velocity dispersion is deduced from computations taking into account the silica photo-elastic

tensor, and the constant propagation variation resulting from the opto-geometrical deformations.

Experimental considerations:

The experimental set up is shown on figure 1. The Mach-Zehnder interferometer is implemented with a York HB

600 polarization-maintaining fiber in order to avoid polarization cross coupling. A high pressure Xenon lamp is used as source and the study is performed over a [650-850 nm] spectral range for which silica chromatic

dispersion effects are important.

Croupe photonique, IRCOM (UMR 6615)

J 23, avenue A.Thomas, 87060 LIMOGES cedex -FRANCEAbstract:

High pressure Xe Lamp

Coupled spectral bandwidth A X = (650, 850) nm

Air delay line Total stroke ALair = 50cm

Prism

Fiber delay line : total stroke 32 cm

Fig. 1: Experimental setup.

165
The field amplitude division at the input of the interferometer and the flux recombination at the output are performed by two beam-splitters. The light source is linearly polarized and injected by means of microscope

objectives into the two interferometric arm inputs, the optical fields being aligned with the fast axes of the fibers. The optical fiber delay line consists of fifteen-meter length of polarization maintaining fiber wound and glued on

a rubber rim. A hydraulic pump drives the rubber rim radial expansion so that the fiber is uniformly stretched. An air delay line consisting of a variable air path by motion of a motorized translation is integrated in one of the interferometric arm in order to adjust the zero group delay. The interferometric mixing available at the output of

the interferometer is spectrally analyzed by means of a prism spectroscope with a linear CCD detector. The phase shift 0(v) between the two interferometric arm can be expressed as:

<I)(v) = Of(v) + (l>o(v) (1)

where O, (v) is the phase shift introduced by the propagation in the fibers and 0„(v) results from the air path

propagation.

The fiber contribution O, (v) may be represented as a first approximation by the sum of a linear term and a quadratic one where as 0„(v) consist in a linear term. For a zero group delay obtained experimentally by

adjusting the air delay line the second order phase shift is only remaining and can be expressed by:

0(v) =

5vL, 5v-

(2)

where 3, .( i=l or 2 ) is the propagation constant, and Li, Li are the fiber length in the two interferometric arms. This quadratic term results from the group velocity dispersion and is fully sufficient to fit accurately the experimental analysis [4]. In order to characterize and scale the chromatic dispersion variation versus fiber stretching two experimental

configurations are compared (figure 2).

EXPERIMENT A EXPERIMENT B

Same equivalent air path

i 4-

->K !

Fiber lengthening under stretching : AL;.

Chromatic dispersion variation : 5v^

t

AL,

-< '' >\<

DC

AddiHonnal fiber AL,

Chromatic dispersion variation

COMPARISON

5v'

Fig. 2: Comparative study between the chromatic dispersion of a stretched fiber and an unstretched one.

* In the first case, (denoted experiment A, figure 2, left side), the phase variation AO^, results from fiber

stretching: The pressure, applied on the optical delay, is increased by step of 0.5 bar over a 0 bar-6 bar span. At

each step, the zero group delay is adjusted. The resulting channeled spectrum is displayed on the linear CCD

detector and the parabolic phase shift is determined as a function of the frequency.

* In the second case (experiment B, figure 2, right side), the phase shift AO„, varies versus the fiber length: The

fiber length of the interferometric arm including the free beam air delay line is modified by replacing the input

fiber labeled "if (figure 1 ). For each replacement, the zero group delay is adjusted and the associated phase shift determined.

Figure 3 reports the two second order chromatic dispersions variation of the two configurations versus the generated air path. The two curves exhibit a linear behavior of the second order chromatic dispersion coefficient. The slope is a=1.03 rad.ps'm ' for experiment A, and a=l,3 rad.ps'm'' for experiment B. Thus, for a given air delay, the additional chromatic dispersion of the stretched fiber remains smaller than the one of an unstretched

fiber.

166
Fig. 3 : Evolution of the second order phase term versus the generated air path.

Theoretical model [5,6,7]

Assumptions : The optical fiber used in the experiment is a polarization maintaining fiber glued on a variable diameter drum. Only one axis of the fiber is used so we consider an equivalent step index fiber which refractive index is deduced from the real index profile of the polarization maintaining fiber. In order to simplify the model, we assume that the drum diameter is enough important to ensure that the experimental configuration is equivalent to a wave guide axial elongation.

An optical fiber under strain suffers opto-geometrical parameters modifications. The deformation vector e can be

written by the following expression :

— V £ — V £ (3)

where Vp is the Poisson coefficient of the silica (Vp=0.17), and £j the axial elongation.

This vector shows clearly that the volume is not preserved, the Poisson coefficient Vp <1. Then, when the fiber is under stress, the core and cladding radius decrease and the index repartition is modified. The waveguide

propagation characteristics determination makes necessary to derive the material refractive index modification

from geometrical deformation. The photoelastic effect describes the relation between the mechanical strain and

the resulting refractive index change in the material. We can define the strain-optic tensor by :

(4)

and the resulting refractive index change Ani

Pii P.2 Pl2

Pl2 P.. Pl2

Pl2 Pll

An, = XP.£j with i = r, 0, z (5)

In order to compute the effective index of the stretched waveguide the following parameters are taken into account: -Modification of core diameter. -Modification of the core and cladding refractive indexes.

In a second phase, this effective index evolution can be fitted by a second order polynomial approximation,

allowing us to express the propagation constant and its successive derivatives. In this approximation the variation of Pij tensor versus frequency are neglected.

In the case of experiment A we express the second order phase term in the stretched fiber by:

l2o ^Ir (6)

where ALc is the axial elongation, L the length of the fiber under testing, Pl and Pc are the propagation constants

in the unstressed and stressed fiber and AO,, the second order phase term variation in the stressed fiber.

167
In the second part of calculation, the phase variation results from fiber lengthening and the second order term

variation can be expressed by :

'ALl (7)

av^ avwhere ALl is the fiber length added in order to have the same group delay than in the first part of calculation. Eq.6 and Eq.7 are plotted in the followmg figure in order to compare the difference of the second order phase term in a stretched fiber and an unstretched one (fig. 4).

Fig. 4: Evolution of the second order phase term for several equivalent generated air delay.

We obtain a slope of 1.29 rad ps" m" for the unstretched fiber and 1.13 rad ps' m" for the fiber under strain. We

can note that the difference is only 10% between the theoretical and experimental values for the stretched fiber, leading to a discrepancy of 30% for the difference between the theoretical evaluation and the experimental

measurement of the second order phase term in a stretched or unstretched fiber. The assumptions introduced in the theoretical development allow a good approximation.

Discussion and Conclusion

The developed model allows simple calculations which gives results in good agreement with the experiment.

The 30 % slope difference between experimental and theoretical stretched fiber can be explain by the relative approximations : The experiment has to be implemented using a polarization maintaining fiber in order to avoid

cross coupling perturbations. As long as only one principal axis of this fiber is fed, our model simulates an index

step fiber that fit the experimental refractive index. Assuming the photo-elastic coefficient to be achromatic, this result can be extrapolated to different spectral

domain. The emerging data on dispersion effects allows to design optical fiber delay lines in the frame of

correlation or synchronization of optical fields. BIBLIOGRAPHY

[1] SIMOHAMED L.M., DELAGE L., REYNAUD F. : "An optical fibre delay line with a 318 mm stroke". Pure and Applied optics, 1996, 5, p. 1005.

[2] SIMOHAMED L.M., REYNAUD F. "A two meter stroke optical fibre delay line ", Pure and

Applied optics, 1997,6, p. 37.

[3] SHANG H. T. "Chromatic dispersion measurement by white-light interferometry on metre-length singlemode optical fibers", Electron. Letter, 1981, 17, 17, p. 603.

[4] REYNAUD F., ALLEMAN J.J., CONNES P. "Interferometric control of fiber lengths for a coherent

telescope array" , Applied Optics, 1992, 31, 19, p. 3736.

[5] LAGAKOS N., BUCARO J.A., LARZYNSKI J. : "Temperature-induced optical phase shifts m fibers". Applied Optics, 1981, 20, 13, p.2305.

[6] BERTHOLDS A., DANDLIKER R. : "Deformation of single-mode optical fibers under static longitudinal

stress". Journal of Lightwave Tech., 1987, 5, 7, p. 895.

[7] RIOUBLANC J. : "Etude d'un systeme de stabilisation passive de la derive en temperature de la longueur

d'onde d'accord des reseaux de Bragg", Ph D thesis, Universite de LIMOGES, 1997.

168
Fiber Dispersion Measurements using

a Supercontinuum Fiber Laser

F. Koch, S.V. Chernikov, and J. R . Taylor

Femtosecond Optics Group, Physics Department, Imperial College,

Prince Consort Road, London SW7 2BZ , UK

Broadband sources operating in the spectral range of 1 to 1.8 jam are of significant

interest for the purpose of characterisation of single mode optical fibers and fiber based

components. Standard measurements in single mode optical fibers like losses, chromatic

dispersion, polarisation mode dispersion are commonly performed with tuneable lasers or

superfluorescent sources in one of the two spectral regions around 1300 nm and 1550 nm.

Recently, we have demonstrated a supercontinuum fiber laser based on a self-Q-switched

Ytterbium fiber laser [1]. The laser

generates a continuos supercontinuum spectrum from 0.8 to

2 |jm with an extremely high

spectral brightness ~ 0.2 mW/nm

within the range of 1.1-1.75 |im

which is for comparison at least

four orders of magnitude brighter

than a single mode fiber coupled

conventional "white-light" source

operatmg m this spectral range. A

^'g""-^ ^ Spectrum of supercontiuum laser source

typical spectrum of the source is illustrated in Figure 1. The laser operates in a self-Qswitch mode generating pulsed with a duration of a couple of nanoseconds at 10-20 kHz

repetition rate. Here we report on applications of the source for measurement of

dispersion characteristics of various types of single mode fibers.

I—

• 1 1 1 1.3 1.4 1.5 Wavelength(ijm)

169
Two different techniques were used to measure the dispersion: interferometric

measurement and time of flight measurement [2]. The interferometric measurements were

undertaken using a fiber Mach-Zehnder interferometer comprising two 3dB-couplers with

an air gap in the reference arm and test fiber typically ~60cm long in the other arm. All

measurements were undertaken in spectral domain using a conventional spectrum

analyser. Representative interferometric measurement patterns are shown in Figure 2. The

positions of the minima , maxima and centre of the interferometric pattern were used to

determine the dispersion of the fiber. The dispersion characteristics of the fiber can then

be calculated either based on a single short measurements or by scanning the length of the

reference arm.

By scanning the length of the reference arm of the interferometer, the centre wavelength

where two arms of the interferometer optically are equal moves as shown in the Figure 2. The group delay as a function of the wavelength follows directly from this measurements,

and the chromatic dispersion can be found by differentiating the curve. This technique

could be used for measurement of dispersion of all types of fiber. In particular, we studied

the dispersion properties of various dispersion compensating fibers(DCF) in the range of

1.1 to 1.7 )im. Some results are shown in Figure 3.

Wavetength(|jn)

1.3 1.4 Wavelength (pm)

Figure 2 Representative interference patterns Figure 3 Measured chromatic dispersion of

three different dispersion compensating fibers

170
The dispersion curve depends strongly on the design of the dispersion compensating fibre. The other important feature of DCF is that the dispersion can strongly be affected by fiber

bending. The effect was investigated using the above measurement technique. Curves (a),

(b) and (c) in Figure 3 show the dispersion curves of the same fiber placed in a coil for a number of bending diameters. The bending effects the value of the dispersion, slope and

also the shape of the curve. The results are well explained by theoretical modelling and

are of importance not only for application but also for design optimisation of dispersion

compensating fibers.

The information about the fiber dispersion can also be obtained from a single shot

measurement of interferometric pattern similar to those shown in Figure 2. By measuring

the period of oscillations in the pattern, it is possible to determine the dispersion,

dispersion slope, zero dispersion wavelength and even dispersion curve in the entire

spectral range. For these "single shot" type measurements, the stability of the

interferometer is extremely important in order to obtain accurate results.

-» I ' I ' i ' I " ' I ' ' I I '

'

1.15 1J0 1.2S 1J0 1.35 1.40 1.45 ISO 155 1 60 1 &5 -10 0 10 20 30 40 Wmlangth(|<n) Time (ns)

Figure 4 Pulse in time domain at 1550nm Figure 5 Dispersion measured by different

techniques

The interferometric technique is suitable for measurements in short pieces of fiber

typically from tens of centimetres up to a few meters. For evaluation of long fibers (from

hundreds of meters to tens or even hundreds of kilometres), the time-of-flight type of

measurements can be implemented using this supercontinuum source. For the

measurements, the spectrum of the source was first sliced using a simple diffraction171
grating-based monochromator with fiber 'input and output. The monochromator was

adjusted to ~ 2-3 nm bandwidth, and could be tuned within the entire spectrum of the

source. A typical shape of the filtered pulse is shown in figure 4. The rise time of the

pulse was less than 0.5 nsec limited by the resolution of the detection set-up included a

fast detector and digital oscilloscope. This resolution was however sufficient to measure

reliably the dispersion of dispersion compensating or dispersion shifted fibers with the

minimum lengths of 200 m and 3 km respectively. As an illustration, figure 5 shows the

dispersion curves measured by the interferometric and time-of-flight techniques in a short

and long pieces of the standard telecommunication fiber. The results show the reliability

of the measurements even in the vicinity of zero dispersion.

In conclusions, we have demonstrated the application of the supercontinuum fiber laser

for measurements of dispersion in various singe mode fibers. The source which is

currently available as a compact diode pumped package, looks very promising for

applications in various fiber characterisation apparatuses.

References:

[11 S.V. Chemikov, Y. Zhu, J. R. Taylor, V.P. Gapontsev: "Supercontinuum self-Qswitched ytterbium fiber laser". Optics Letters, Vol. 22, No. 5, March 1, 1997, p. 298-300

[2] Leonard G. Cohen: "Comparison of single-mode fiber dispersion measurement

techniques", Journal of Lightwave Technology, Vol. LT-3, No. 5, October 1985,

p. 958-966

172
PLANAR COMPONENTS FOR DENSE-WDM

J.Amin, R.O.Maschmeyer, A.Beguin, C.Lerminiaux*

,

D. Trouchet , N.Boos* and C.Nicolas*

Coming Inc. Coming, NY 14831

Tel: (607) 974 3042 Fax: (607) 974 3518 Email: aminjfljconiing.com

(*) Coming S.A

7bis, Avenue de Valvins, 77210 Avon, France

The seemingly limitless demand for bandwidth created by the phenomenal growth of

industry sectors such as Intemet and high-speed data has spurred service providers to push for new ways to expand their fibre-optic cable capacity. Affordable bandwidth in

network backbones is becoming a scarce commodity, with many fibre networks in the US

operating close to their maximum overall capacity. In order to keep up with this everrising demand for capacity, carriers are installing wavelength-division mutliplexed

(WDM) networks, where multiple channels located on separate wavelengths are simultaneously transmitted down an optical fibre. System houses are currently offering 8

and 16 wavelength solutions to the capacity dilemma, each wavelength separated by

lOOGHz and delivering 2.5 Gbs'^ of bandwidth, for an aggregate of upto 40 Gbs"\ and

systems capable of carrying OC-192 line rates (lOGbs"^) have also been mstalled. With

no sign of abatement in user demand for bandwidth, there is already talk in the industry

of installing 40- and 80-channel systems by mid- 1999. In addition, creating reliable

network infrastructures for WDM has led to a surge in R&D on optical add-drop

multiplexers and optical cross-connect systems, which will enable all-optical networks to perform at a higher level than electronic networks.

The adoption of these dense- WDM (DWDM) functions in network architectures requires,

in addition to highly advanced gain- flattened amplifiers, the availability of key optical

components such as multiplexers and demultiplexers, dispersion compensators, switches,

and tunable filters. Planar waveguide devices have recently started playing a role in

WDM, in particular for passive optical components required in multi-wavelength

systems, where the planar configuration offers the advantage of compactness through

manipulation of all wavelengths on a single chip. The most attractive of such passive

planar devices is the phased array demultiplexer, which is currently finding commercial

application in 8- and 16-channel networks. In addition, laboratory demonstrations of 40-

channel devices have also been made. Other commercially available planar WDM

devices include switches and switch arrays, generally composed of Mach-Zehnder

elements or digital Y-branches. Lattice filters for gain-flattening in amplifiers and also

for providing tunable dispersion compensation have also been explored in various

research laboratories. In this presentation, the state-of-the-art of planar components and

their applications for DWDM will be discussed, highlighting key measurement issues in

the characterisation of these devices.

173
174
2.5 Gb/s PRBS Extinction Ratio "Eye Diagram"

IVieasurements of Biased and Bias FreeTM

Lithium Niobate OC-48 Modulators

by: David Williams, Adnan Akhtar, Dan Attanasio, David R. Maack

and Greg McBrien

, of

Uniphase Telecommunication Products

Electro-Optic Products Division

1289 Blue Hills Avenue

Bloomfield, CT 06002

INTRODUCTION

The rapid entry of narrow band WDM technology into today's telecommunication systems has

introduced a number of new components into the hardware infrastructure raising issues of

which measurements, measurement procedures and pass/fail criteria to use in reliability and

qualification testing. With many of these components, industry standard measurement

techniques have not yet been established. One of these, lithium niobate 00-48 modulators, fall into this category of new components without a history of measurement pedigree. Fortunately,

standard 2.5 Gb/s Pseudo Random Binary Sequence (PRBS) Extinction Ratio "Eye Diagram"

Measurements with the proper modified procedures and setup can accurately predict how well

this component will operate in a system.

Lithium niobate modulators are complex components that combine optical, electrical and RF

functions. Each of these functions have a number of intrinsic measurable properties which

when combined, fully characterize the modulator's overall performance. Unfortunately, many of

these are strongly correlated such that a simple criteria for each separate parameter is inadequate to properly define system acceptability. The eye diagram, on the other hand,

measures the extrinsic performance of the modulator as the system would see it by completely

integrating the intrinsic properties.

MODULATOR TYPES

Complicating eye diagram measurement procedures for lithium niobate modulators is that there are two different types of modulators ~ bias ready and Bias FreeTM. The main differences are quadrature bias methods and internal polarization properties. In order for the modulator to properly operate in the system, it must be held at quadrature. In the Bias FreeTM type, the

modulator is set to quadrature through internal manufacturing techniques. With the bias ready

type, the modulator is set to quadrature by application of an external DC bias voltage that is determined through a complex control loop. The Bias FreeTM type is highly polarized, only

allowing the TE state of polarization to propagate while the bias ready type passes both states,

TE and TM, with the TE state being dominant.

THE EYE DIAGRAM MEASUREMENT

175
The eye diagram measurement is performed from the perspective of a digital transmission

system with emphasis on system performance rather than single component performance. The

key system performance parameters derived from the eye diagram are extinction ratio, rise and

fall times, Q, jitter, and crossing percentage. From a pass/fail criteria perspective, extinction

ratio is the most important of these system performance parameters. The higher the extinction

ratio the more margin the system has to overcome added noise in the communication link.

Extinction ratio is the ratio (in dB) of the average optical energy in a logic one level Er(1) to the

average optical energy in a logic zero level Er(0). The average optical energy is measured by

first defining a masked off area centered on each logic level of the eye diagram. The masked

off area is dimensioned to conform to the SONET eye diagram mask for OC-48. Using the

vertical histogram mode measurement features of the signal analyzer, the average value of

each logic level is determined within the masked off area.

The measurement equipment's dark state is a critical parameter that will degrade the extinction

ratio measurement if not accounted for. This dark state is an electrical offset generated by the

detector and signal analyzer with the laser source off. In order to correct for this electrical offset

, an average baseline correction (Cbase) value is determined using the mask and vertical

histogram method. The average baseline correction is subtracted from each logic level's

average value before extinction ratio is calculated.

Extinction Ratio = 10 logio [(Er(1) - Cbase ) / (Er(0) - Cbase)] dB

LN Modulator Detector

DFB

Laser Input Output

RF DC

Active

Bias

Power

Supply Driver

2.4883 Gb/s PRBS

Signal

Analyzer

Pulse

Pattern Generator

Figure 1 Block Diagram of Eye Diagram Test Setup

EYE DIAGRAM TEST SETUP

The digital transmission system consists of a CW DFB laser, lithium niobate (LN) modulator,

modulator driver, detector and communications signal analyzer (see fig. 1). To test system

performance, a transmitted data stream consisting of a pseudo random binary sequence of

2^~^-1 bit length is used. Each component of the digital transmission system has some

contribution to the accuracy and repeatability of the eye diagram measurement.

176
The DFB laser is pigtailed with PM fiber with the laser's TE mode coupled to the fiber's slow

axis. It is important that the output polarization extinction ratio be high enough to minimize

optical propagation in the TM mode. A poor polarization extinction ratio will appear as

attenuation in the highly polarized Bias FreeTM modulator. This attenuation will effect the overall

sizing of the displayed eye diagram, but have no effect on the extinction ratio measurement as

long as there's sufficient power at the detector. For the bias ready device which supports the

TM mode, a poor output polarization extinction ratio will degrade the extinction ratio

measurement by increasing the average pcrwer in the zero logic level.

The modulator's optical output is a single mode (SM) fiber which is terminated in the detector.

The critical issue with the detector is not to exceed the maximum peak input power of the

detector. Eye diagram waveforms can be distorted by an oversaturated detector. A constant

value of optical power should be used with one note that optical power of a modulator biased at quadrature is 3dB down from the peal< optical power during modulation. Also the detector's

3dB bandwidth should be three times the bit rate that is to be measured.

The modulator driver' s RF output is connected to the modulator's RF port. The modulator has

to be driven precisely from the "Off' state to the "On" state in order to get the best eye diagram

performance. The drive voltage required is determined by the modulator switching voltage, Vtt.

If the drive voltage is too high or too low the eye diagram logic levels will be corrupted as shown

in the underdriven case in figure 2. The thickening of both logic levels is primarily due to the

underlying electrical reflections generated by non-ideal modulator S11 and modulator driver

S22. When the drive is set to the optimum point most of this waveform structure is eliminated

due to the "filtering" function of the sinusoidal electro-optic transfer unique to the lithium niobate

modulator.

For the bias ready modulator, the phase setting is critical when performing the eye diagram

measurement. An improper phase setting during the eye diagram measurement will degrade

the measured extinction ratio. An example of a degraded eye due to the phase set significantly

above quadrature is shown in figure 3.

CONCLUSION

The evolution of the measurement techniques and experiences presented in this paper,

provide a foundation for accurate and repeatable eye diagram measurements of lithium niobate

modulators. It is important to remember that the eye diagram measurement is a system

measurement with all components contributing to the final measurement. With attention paid to the measurement techniques and subtleties, one can achieve the best eye diagram

performance using a lithium niobate modulator (see figure 4).

REFERENCES

1. K. Smith and G. Hetu, "Discussion of Extinction Ratio Measurement," Tektronix, Inc., (1991)

2. Bellcore , "Synchronous Optical Network (SONET) Transport Systems: Common Generic

Criteria", GR-253-CORE, Issue 2, (1995)

3. M. Hart, C. Duff, S. Hinch, " Firmware Measurement Algorithms for the HP83480 Digital

Communications Analyzer", Hewlett Packard Journal (1996)

177
-1 i . 5fnV 21. 95ns 22 . Sns

Figure 2 - Underdriven LN Modulator

38. 5mVtrlg'd

11 . SmV.

M. Sft.

gl 95ns: Figure 3 - Improper LN Modulator Phase Setting22. 5ns

-i 1 . 5cnV- 21 . 35ns 5Sp s^ti I w 22, 5ns

Figure 4 - Optimized LN Modulator Eye Diagram

178
Dependence of the composition of InAsi.xSbx on the mole fraction of In in the

melt using LPE.

Vishal Chatrath, S.J.Chua and G.Karunasiri

Center for Optoelectronics, National University of Singapore, 10 Kent Ridge Crescent, Singapore

1 19260

DrY.Mao

Applied Physics Division, School of Physics & Materials, Lancaster University, Lancaster LA

4YB

Abstract: The liquid phase epitaxial technique was used to grow InAsSb on the Gasb substrate using Sb as a solvent. This study departs from the traditional approach where the mole fraction of the group III component (In) was kept constant while varying the ratio between the group V components (As, Sb) to change the value of "x"' in InAsi.xSb^ . In our study we were able to achieve a variation in "x" by keeping the ratio of group V components (As

and Sb) constant, while varying the In mole fraction in the melt. With this approach a wider range of temperature can be used for the growth, and layers with both positive and negative lattice mismatch can be obtained without

any indication of substrate erosion. To the best of the author's knowledge this is the first study of this kind.

1. INTRODUCTION

There has been a growing interest in the III- V ternarv' alloy lnAsi_xSbx as a material for the fabncaiion of as a maieriai for the fabricalioii of

infrared light emitting diodes and detectors for use

in spectroscopic analysis, optical fibre communication and defense applications. The InAsi_

xSbx is regarded as a promising material because of

its room temperature energy gap covering 3-5 |j,m range of the spectrum when grown on near lattice matching InAs or GaSb substrates. In this work we investigate the effect of the variation of the mole fraction in the melt of a group

III component (In) on the composition of the group

V components (As. Sb) in the epita.xial layer.

Previous work in the growth of this material was

confined to varying one of the group V elements (As.

Sb) in order to change the composition of the

epitaxial layer. Hence, to the best of the author's knowledge this is the first attempt to understand and

explain the influence that the group III component

can have on the composition of the ternary.

2. EXPERIMENTAL PROCEDURES

High quality lattice-matched epitaxial layers

of n-liiAsi.^Sbx (where x is in the vicinity of 0.09) were grown by LPE on Te doped GaSb stibstrates (EPD<5000) supplied by ATRAMET Inc. The

horizontal sliding boat technique was used and

growth from Sb solution was employed to avoid

substrate erosion. Growth was carried out at three

different temperatures of 565°C, 567°C and 570°C

using diifereni mole-fractions in the meit. Smooth mirror-like epitaxial layers of

InAsi.xSbx of controllable uniform thickness were

obtained under close lattice matching conditions by

using 2-7 °C melt super-cooling. The thickness of

the epitaxial layer used in the study were typically 4- 6 |j.m and cleaved stained samples showed no

evidence of any substrate erosion. After the epitaxial

growth the samples were analyzed under the Philips

DCD3 double crystal X-ray diffractometer (DCDX).

From the peaks obtained, the lattice mismatch

between the substrate and the epitaxial layer can be

calculated from which we could deduce the composition of the epitaxial layer.

3. RESULTS AND DISCUSSION

The width of a Bragg reflection inevitably indicates the crystal quality of the grown layer. The

full-width-at-half-maximum (FWHM) of a perfect

reflection is calculated as around 10 by B. E. Warren since a perfect GaAs (004) reflection is about 8-9 . This is used as the criterion when

evaluating the crystal quality.

Figure 1(a) is the DCXD curve for the epilayer grown. Four peaks were observed, two each

for the substrate and the epitaxial layer. The reason

179
for the two peaks is the incompatibihty of the GaAs

reference crystal in the DCXD with the material

under analysis. The two peaks to the left correspond

to the substrate and the magnitude of the split (~30

arc sec.) is typical. The two peaks to the right

correspond to the epilayer. If the epilayer peaks are

to the right of the substrate peaks then it corresponds

to a positive lattice mismatch and vice versa. If the

lattice matching is too close then the peaks tend to overlap.

Under these conditions curve fitting is used

to see the various components as shown in Figure

1(b). From the figure the percentage lattice mismatch can be calculated. Further, applying

Vegard's Law, the composition of the epilayer can be

obtained and the results are plotted in Figure 2.

1400

-1200

1000

800

600

40O

-7800 -7700 -7600 arc seconds

(a)

InAsSb/GaSb

DCXD SI 5

-7400 -7300 -7200 arc seconds

(b)

In mole fraction in the melt the Sb composition in the melt, increases, thus edging out As. This effect can be understood by close reference to the thermodynamics involved in the LPE grovW;h. Fig 3 illustrates the density variation in LPE solution. This

would mean that the denser components in the melt

tend to remain towards the bottom of the well,

edging out the less dense ones

O 0.02

0.210 0.212 0 214 0,216 0.218 0.220 Indium mole fraction in tine melt

(a)

0,212 0 214 0.216 0.218 0.220

In mole traction In the melt

(b)

Fig 2 (a) Variation of Sb composition in the epitaxial layer InA,Si. xSbx with change in the mole fraction in melt, (b) Variation of lattice mistnatch with increasing In mole fraction in the melt.

The relative densities of the of the components are as

follows:

Fig. 1(a) Typical DCXD curves, (a) shows a epitaxial layer of a

larger lattice constant than the substrate, (b) shows a closely lattice matched structure. Here the curve has been resolved to show the individual peaks, A curve of the Sb composition in the solid versus the mole fraction of In in the melt is given in Figure

2(b). The result suggests that with the increase in the

Arsenic (As):

Antimony (Sb): Indium (In):

5.72 gmJcnv'

6.618 gm/cm^

7.3 1 gm/cm''

180
TOP OF

SOLUTION

SUBSTRATE 0 ^r p

Fig. 3 Density variation in LPE solution.

r

As more In is added, it sinks to the bottom

thus edging out the other two components namely As

and Sb. But tliis effect is more pronounced for As

than Sb because of Sb being denser than As. Thus in the region of the meU close to the substrate, the

fraction of Sb with respect to As increases. This means there is more Sb available to occupy the group

III sites, thus increasing the composition of Sb in epitaxial layer. Since the lattice constant of the

epitaxial layer is a function of tlie composition, we

can also see a variation in the lattice parameters.

This is shown Figure 4. But increasing tlie In mole

fraction beyond a certain level a which the lattice mismatch tends to remains constant. This effect can

be attributed to the fact that LPE growth being an

equilibriiun process, the substrate crystal does not

tolerate the epilayer to grow beyond a certain level of

lattice mismatch. Doing growths at different temperatures further strengthened the validity of this theor\ . When the growth was carried out at a higher

temperature of 570°C the same trend was seen with

the increasing In mole fraction (Fig4).

in the decrease in the composition of As. This is in support of the above-proposed theory and can be

explained as follows. With the increase in temperature the convection currents played a

stronger part which reduced the tendency of the In

(the heaviest component) to displace As (the lightest

component) from the lower reaches of the melt

solution. That means that more As was able to come

in contact with the group V sites more readily and

thus its composition in the epitaxial layer was

increased.

Growth at 565°C also supported this hypothesis. Here the convection effect was lowered

due to the lowering of the temperature which

increased the chance of In displacing As from the

vicinity of the substrate, resulting in an increase of

Sb composition in the epitaxial layer. Unfortunately

the growth window at 565°C is very narrow making

it difficult to do a growth with a different mole

fraction.

4. CONCLUSION

A new model for the LPE growth has been

demonstrated. The effect of the density of the components incorporating the melt has been shown

lo be quite significant. This heralds a new area on which further study can be carried out.

References:

1. Y. Mao and A. Krier, J.Cry.stal Growth. 133(1993) 108-116.

2. Y. Mao and A. Krier, J.Phys. Chem. Solids Vol. 56 No. 5, pp. 759-766, 1995.

3. T.C. Ha.senberg et al. Electron. Lett. 31(4), 275 [16 Feb 1995|.

4. H.K. Choi and G.W. Tumer. Appl. Phys. Lett. 67(3). 332 1 17 July 1995].

5. H. Lee. et al. Appl. Phys. Lett. 66(5), 1942 | lOApril 1995]

0.18

0 16

0 14

0.12

0.10

o.oe

0 OG

0 04

0 02

0 00

-0 02 -

-0 04 0 06 - 0.210 0 212 0 214 0 216 0 218 0 220 Mole fraction of Indium In the melt

Fig.4 Variation in the lattice mismatch of InAsSb with respect to the GaSb sub.strate under different temperature conditions.

The interesting thing to note is that when

the growth temperature was decreased the incorporation of Sb in the layer increased resulting

181
182
Active Mode-Hop Suppression in External Cavity Lasers

Shigenori Mattori, Takanori Saitoh, Shigeru Kinugawa, and Koichiro Miyagi

Research Laboratory, ANRTTSU CORPORATION

Abstract We have demonstrated active mode-hop suppression in external cavity

semiconductor lasers including a wavelength-selecting diffraction grating, based on closedloop control to nullify the difference between the oscillation wavelength and selected

wavelength due to the change in diffraction angle. Our approach can be adopted for most

Littman-type external cavity lasers with simple attachments; it will be useful for continuous

wavelength sweeping and for long-term wavelength stabihzation, such as pump sources of

atomic frequency standards.

1 . Introduction

Recently, optical frequency measurement and control are becoming very important in optical

communications, especially for WDM and coherent systems. Also, these techniques are needed in spectroscopy and laser coohng. External cavity semiconductor lasers (ECLs)^^^ are used widely for such appUcations because of the wide tuning range and narrow spectrum line width, as well as the

simple structure. However, mode-hopping sometimes occurs in ECLs, which limits the

applications.

Many improvements have been made from the aspects of mechanics and system tuning.

However, such approaches have never been completely successful because suppression of modehopping (MHS) requires severe conditions. To solve these difficulties, we investigated active MHS

based on closed-loop control.

2. Cause of Mode-Hopping and Conventional MHS

ECLs usually consist of an optical resonator including a laser diode (LD) with an anti-reflection

film on the end facet, and a diffraction grating. The oscillation wavelength is determined by two

factors: the resonance modes of the cavity, and the band selected by the diffraction grating. We

call the center wavelength of the selected band "selected wavelength," and the wavelength difference

between the oscillation wavelength and the selected wavelength "detuning." Mode-hopping occurs

if the detuning becomes much larger than half of the mode spacing. Therefore, MHS requires

control of these two factors. Conventional MHS techniques are classified into two types: (1) suitable and accurate mechanical design, and (2) simultaneous precise control of the above two factors. In normal

Littman-type ECLs , setting a suitable mirror rotation pivot cancels detuning at any rotation

[31 angle . In other words, this approach reduces the two factors to one by mechanical linkage.

However, this approach has severe demands for implementation, initial adjustment, and maintenance,

because the pivot position is critical, typically within 50 \xm for a continuous wavelength sweep of

10-nm span in the 1.5-^m region. To use approach (2), the relationship between the wavelength

and the two factors must be determined and saved at initial tuning. Nevertheless, the data saved in

183
any initial table will eventually become partly unusable due to long-term drift. From the control viewpoint, conventional MHS techniques are based on open-loop control. In

other words, all behavior is based on the initial conditions, so the performance is generally sensitive

to incomplete initial adjustment, environmental change, and any kind of drift.

3. Principle of Active MHS

To solve this difficulty, we propose active MHS based on closed-loop control to nulhfy the

detuning. The most important item in achieving this goal is detuning detection.

Figure 1 shows the (exaggerated) change in the diffraction angle caused by detuning on a Littrowtype ECL, where X is oscillation wavelength, and Xg (fixed in figure) is the selected wavelength.

When there is no detuning (A=Ag), the diffracted beam returns exactly on the optical axis of the

incident beam (Fig. 1 (b)). Assuming that the axis of the incident beam is geometrically nonvariant for wavelength change, at detuning, the diffracted beam tilts from the axis as shown in Figs.

1 (a) and 1 (c). The change in the diffraction angle is observed as beam displacement using a beam sampler and a screen (or CCD array, etc.) in the far field as shown in Fig. 2 (a). The displacement is almost

proportional to the detuning because the change of the diffraction angle corresponding to the

detuning is usually smaller than 0.01°. To achieve simple detection, we imagined two photodetection areas and their input powers 5], ^2, as shown in Fig. 2 (b). We defined the normalized

power difference of both sides (NPD) as {S2-S{)/ {S\+S-t). For a Gaussian beam, figure 2 (c) shows

the NPD versus detuning normalized by the resolution bandwidth of the grating. The upper

abscissa shows the displacement normalized by the beam diameter in the far field. On the other

hand, for actual Littman-type ECLs, the ratio of the resolution bandwidth to the free spectral range

(FSR) of the cavity is less than 20; mode-hopping occurs if the detuning becomes larger than half of

FSR, namely, larger than 0.025 of the resolution bandwidth. From Fig. 2 (c), a detuning of 0.025 of

the resolution bandwidth gives an NPD of about 0.05, or 5% of the full scale, which is easy to detect. Consequently, a small change in detuning that never causes mode-hopping can be detected by

measuring NPD in most Littman-type ECLs.

4. Experimental Setup

Figure 3 shows a diagram of the experimental setup. The ECL includes an additional beam

sampler and PZT actuator. For detection, the beam sampler leads the beam outside the cavity, and

has a reflectivity of 20%. The PZT actuator can translate the mirror in the 6-|im range, independently of the rotation.

The FSR of the ECL is 2 GHz (16 pm) at 1520 nm. The estimated resolution bandwidth is about

20 GHz (160 pm). The micrometer driven by a stepping motor rotates the mirror and changes the

wavelength; the ratio of the wavelength change to the micrometer displacement is approximately

-3 pm/0.25 |um. To achieve frequent mode-hopping, the rotation pivot is not at the best position for continuous wavelength sweeping.

The detector consists of a lens and a CCD camera, because the displacement can be monitored

184
directly. (Use of a bisection-photodiode would simplify the actual detector.) The lens focuses the

reduced far field pattern on the focal plane and the CCD camera detects the beam spot image. A

computer converts the image data into the NPD. In addition, the control system can adjust the

length of the PZT actuator to nullify the NPD. Feedback control matches the oscillation

wavelength with the selected wavelength by changing the cavity length.

5. Experimental Results

First, we observed the relationship between the NPD and wavelength. Figure 4 shows the NPD

(dots) and wavelength (hne) in the acquisition 'order as wavelength was decreased and then increased over an 0.8-nm span. The NPD jumps at several points (triangles), also the wavelength hops over about 16 pm at the same points. We confirmed similar results at other measurements including

about 50 mode-hops at each wavelength change of 0.16 nm on average. These results show that demning can be estimated by observing the NPD. The averages of the NPD (dashed lines) are

clearly different depending on the direction of the wavelength change. We think this is due to cross

[4] saturation between resonance modes . However, the hysteresis is favorable because it expands

the tolerable detuning range without mode-hopping.

Next, we demonstrated active MHS. Figure 5 (a) shows the wavelength (diagonal curve) and its increments for each step (dots) measured under active MHS control. The wavelength increments of

about 3pm are determined by the micrometer displacement step (-0.25 |im), except at the beginning

of the rotation shown on the right. The figure clearly shows that there is no mode-hopping over the

1.2-nm span. The span, which is restricted by the movable range of the PZT actuator, is 7.5 times

the average span without control (0.16 nm). In contrast. Fig. 5 (b) shows the wavelength and its increments measured without active MHS control; several dots near 16 pm indicate the occurrence of mode-hopping.

6. Conclusion

We propose an active MHS control technique for ECLs based on closed-loop control to nullify

detuning due to the change in diffraction angle. We demonstrated detection of the angle change

and MHS in a Littman-type ECL with simple attachments. Our approach can be adopted for most

Littman-type ECLs; it will be useful for continuous wavelength sweeping and for long-term

wavelength stabilization, such as pump sources of atomic frequency standards.

References

[1] For example, M. W. neming and A. Mooradian, IEEE J. Quantum Electron., vol. QE-17, No. 1, 44-59,

1981

[2] K. C. Harvey and C. J. Myau, Optics Lett., vol. 16, No. 12, 910-913, 1991

[3] H. Lefevre et al. (Photonetics S.A.), US Patent No. 5594, 1995

[4] M. Yamada, IEEE J. Quantum Electron., vol. QE-19, No. 9, 1365-1380, 1983

185
LD Collimator Grating

(a) A< Ag i r I I

(b) A = Ag >

(c) A > Ag

Fig. 1 Illustration of diffraction angle in ECL due to detuning

CoUimator ^^^^ Grating Sampler / ^ LD ''^ ^

(a) g-

(b)

Diplacement / Diameter

-1 -0.5 0 0.5 1

i .U r-TT T ' I TT ' t ' II ' '

(c)

1 -0.5 0 0.5 1 Detuning / RBW

Fig. 2 (a) Illustration of beam displacement,

(b) detection powers in two areas, (c) calculation of NPD versus normalized detuning

( RBW; resolution bandwidth of the grating )

ECL Display

Mpor \ Grating 1100/mm

Attenuator ~60dB

Lens

f=50mm CCD

Pivot

to Wavemeter

Computer

0 100 200 300 400 500

Acquisition No.

Fig. 4 Measured NPD and wavelength in order of acquisition.

-50 -25 0 25 50

Micrometer displacement [ fim ]

(a) with control

20

- 15 cs

G

u

0

to J 5\- I--: -NJ-0.4_

0.8

&3 0.4 <D

TO

-0.8

Fig. 3 Experimental setup

-50 -25 0 25 50

Micrometer displacement [ )im ]

(b) without control

Fig. 5 Wavelength increment for each step

of micrometer displacement (dots)

186
AUTHOR INDEX

Abbott, John 119

Akhtar, Adnan 175

Amin, J 173

Artiglia, M 19

Aso, Osamu 139

Attanasio, Dan 175

Auguste, J.L 165

Barlow, A.J 23

Beguin, A 173

Benson, Jerry M 123

Bigo, Sebastien 77

Billington, Robert S 123

Blondy, J.M 165

Boos, N 173

Brendel, J. 35

Brener, 1 163

Brodzeli, Z 149

Canning, John 143, 149

Caponi, R 19

Cattelan, Susanna 65

Chang, Richard K 73

Chatrath, Vishal 179

Chbat, Michel W 77

Chernikov, S.V 169

Chua, S.J 179

Corbeil, S 53

Corsi, Fabrizio 105

Craig, R.M 5

Cyr, N 45

Dainese, M 159

Dal Forno, A.0 97

Ducharme, Alfred D 131

Dueser, Michael 135

Ellison, J.G 109

Fontaine, N.H 61

Franzen, D.L 127

Galtarossa, Andrea 105

Geiser, C 19

Gilbert, S.L 5 Gisin, B 49

Gisin,N. 19, 35, 49, 97, 101

Gripp, Jiirgen 153

Hackert, Michael J 113

Hale, P.D 5 Rotate, Kazuo 91

Huttner, B. .. 19, 49, 97, 101

Ives, David 15

Joindot, 1 19

Jones, Timothy C.E 123

Kapovits, Adam 31

Karunasiri, G 179

Kawanishi, Satoki 87

Kinugawa, Shigeru 183

Koch, F 169

Krug, Peter A 143

Lee, D.D 163

Leppla, Ralph 27

Lerminiaux, C 173

Lovisolo, F 159

Maack, David R 175.

Mackechnie, C 23

Mao, Y 179

Markos, Constantine T. 131

Marradi, Gisella 65

Maschmeyer, R.0 173

Mattheus, Arnold 27

Mattori, Shigenori 183

McBrien, Greg 175

Mitra, P.P 163

Miyagi, Koichiro 183

MoUenauer, Linn F 153

Muller, A 35

Namihira, Yoshinori 83

Namiki, Shu 139

Nelson, John C.C 123

Nicolas, C 173

Nyman, Bruce 1 Onishi, Tsuyoshi 135

O' Sullivan, M 53

Palmieri, Luca 105

Parker, A.J 57

Passy, R 97, 101

Pepeljugoski, Petar 119

Pereira da Silva, J. A 97

Peretta, Pier Giuseppe .. 65

Philen, D 163

Poon, Andrew W 73

Potenza, M 19

Raine, K.W 57

Reecht, J 101

Reed, W.A 135

Reynaud, F 165

Rioublanc, J 165

Rossaro, A 159

Saida, Takashi 91

Saitoh, Takanori 183

Schiano, M 19, 41, 159

Schlager, J.B 23, 127

Sergienko, A 35

Shake, Ippei 87

Siddiqui, A.S 109

Simohamed, L.M 165

Stepanov, D.Yu. ... 143, 149

Tadakuma, Masateru .. .139

Takara, Hidehiko 87

Tatum, Jim 119

Taylor, J.R 169

Thomson, D.J 163

Trouchet, D 173 von der Weid, J.P. ... 97, 101

White, W.R 135

Williams, David 175

Williams, P.A 23

Wittmann, Ronald C 69

Wyntjes, Gerry 131

Wysocki, Paul F 9

Xavier de Barros, M.R. .. . 97

Yamabayashi, Yoshiaki ... 87

Young, M 61, 69

Zbinden, H 35

187
,1
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